Summary/specs at the bottom. This is concerning the G502 LIGHTSPEED, not the wired G502.
Ah, the wireless battle of the ages: the G900 Chaos Spectrum vs G502 LIGHTSPEED. These are both high-quality wireless gaming mice, and many people are split between the two. I'm going review multiple different traits of each mouse and decide which mouse has the advantage in that category, then make a general assessment. Enjoy!
Grip
These two mice have antithetical grips. The G900 has a true ambidextrous grip, while the G502 has an extremely right-handed one. The G900 has an ambidextrous, versatile, grip. It can work for fingertip, palm, or claw. The rubbered sides of the G900 are great to hold the mouse in any position, something that the G502 doesn't have. The G502 is the perfect shape for a palm grip. Although I complained in my review that my hand got sweaty, that was because my fingers are relatively long compared to my palm. Your hand slides into the palm grip like it's lubricated. Although most gamers use a palm grip, the G900 is more versatile for any grip.
Advantage: G900 Chaos Spectrum
Hotkeys
Both of these mice have great hotkeys. They actually have the same amount at 9 (I do not count pushing the scroll wheel left/right as it is extremely impractical); just placed in different spots. The G900 has completely removable hotkeys on the sides that can be placed or reprogrammed in any configuration; additionally, it has two DPI buttons in the middle of a mouse. The G502 has two hotkeys on the left side and a sniper button. It additionally has the DPI buttons on the left side of the left click as opposed to the middle. The G502 also uses the middle button as a battery life checker by default. The removable hotkeys from the G900 and the sniper button from the G502 balance each other out; it's a matter of preference and which types of games you play.
Advantage: TIE
Sensor
This is another very close battle, and I think I have to end it in a tie. Even though the G502 is capable of slightly higher DPI, I'm not sure anyone uses 16k DPI. At that point, the precision would be too exact to detect. If you just wanted higher DPI to make your mouse move faster, you could just turn up the sensitivity. DPI in and of itself is mostly a marketing gimmick and should be mainly discounted unless your mouse is not capable of 7000 DPI, the highest most gamers go. Both these mice have extremely accurate sensors, but the advantage narrowly goes to the G502 for a slightly more accurate sensor.
Advantage: G502 LIGHTSPEED
Weight
The G900 clocks in at a mediocre 106 grams, while the G502 is 114. So, the G900 is a little bit lighter, although neither mouse could be called an ultralight mouse by any standard. The weight of the G502 wired was 121 grams. How Logitech got the G502 LIGHTSPEED's weight to 114 is an interesting bit of trivia: They thinned the outer shell of the mouse to dangerous levels, but then stabilized it by adding an internal endoskeleton. Lastly, the G502 has 6 different weights you can use to configure the mouse weight, while the G900 has one. You could choose whether you want the lightest possible, or are fine with weight but want more customization options.
Advantage: TIE
Battery Life
Neither mouse uses batteries (yay for the environment), so their battery life can't be too long. With lights on (because everyone uses RGB), the G900 has a battery life of 24 hours and the G502 has a battery life of 48 hours. Although the G502's life is twice as long, since you can easily plug each mouse into a standard micro-usb cord, I think the battery life above 14 hours is generally irrelevant. However, if you can't muster the effort to stick the cord into your mouse at least once or twice a week, I guess you would want the G502...?
Advantage: G502 LIGHTSPEED
Customizability
This category focuses on how much you can tailor your mouse to you. Both mice use LGS, and have a big range of customization options in terms of RGB. The hotkeys are also all able to be individually programmed. Even though these mice have similar key features, I think I needed to throw some light on one of the most extraordinary features of the G900: the removable side buttons. These are completely removable by virtue of magnetism. If you remove both side buttons from a single side, you can cover up the switches with an included hotkey cover. This modularity makes it possible for the mouse to be more beneficial in many use cases. For example, if you're doing day-to-day tasks on your computer and don't want to accidentally hit your hotkeys, you can remove them entirely. Additionally, this solves an inordinately prevalent problem: hotkeys on both sides of a mouse. On many ambidextrous mice, there are hotkeys on both sides to benefit possible left-handed users, or those who can hit hotkeys with their ring finger. However, this can be irritating for users who don't make use of all four hotkeys, because they end up accidentally hitting hotkeys with their ring finger. Now this leads to yet another problem: companies make ambidextrous mice, but only put hotkeys on one side. This even more mind-boggling, because that defeats the purpose of an ambidextrous mouse! Anyway, the removable side buttons can solve all of those problems. The weight customization, however, is a small advantage the G502 has because of more weights. Nonetheless, I'd have to give the advantage to the G900 because of those side buttons.
Advantage: G900 Chaos Spectrum
Value For Price
This is where the G900 annihilates the G502. You have seen so far, the mice are pretty evenly matched. You might be leaning towards the G502 because of the customizable weight tuning, or the extra side button. However, you'll probably be appaled to hear that the G502 LIGHTSPEED costs more than twice as much as the G900. The G900 comes in at a robust $69, but the G502 comes in at a substantial $150. Notwithstanding the occasional $30 sale, the G502 is eons above the G900 in price while being similar in characteristics.
Advantage: G900
Summary
I believe that the G900 Chaos Spectrum is a superior choice of wireless gaming mouse over the G502 LIGHTSPEED. It is more customizable, making it suitable for many different people or can adapt to a player's use cases over time. It has virtually the same sensor as the G502, more customizability, better weight, an adequate battery life, and of course better value for money. Now, if these mice were the same price, I probably would have recommended the G502, but the cost of these two mice really withered the G502's chances.
Key Specs Comparison
Connection: Wireless
Battery life: 24 Hours
Buttons: 9
Shape: True Ambidextrous
OS Compatibility: Mac 10.6+ and Windows 7+
Weight: 107g
Weight Tuning: 1x 10g
Price: $69
DPI: 200-12,000
RGB: 2 zones
Sensor: PMW3366 Optical
Connection: Wireless
Battery Life: 48 hours
Buttons: 9
Shape: Right-handed ergonomic
OS Compatibility: Mac 10.6+ and Windows 7+
Weight: 107g
Weight Tuning: 4x 2g, 2x 4g
Price: $150
DPI: 200-16,000
RGB: 2 zones
Sensor: HERO 16K Optical
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