In this review I'm going to go over comparisons of the two mice, from size, click + scroll wheel feel, to ease of use of their respective software. For the TLDR, skip to the bottom. Apologies in advance for the lack of nice-looking photos, my G502 has seen much, much better days.
1. Size
For my small handed people out there looking for an alternative to the G502, I am happy to say that the Basilisk is it. The two mice feel almost identical, the Basilisk is the tiniest bit wider and shorter, but still incredibly comfortable. I prefer the Basilisk's size personally, as I feel that the slightly shorter length and positioning of the two side buttons + clutch (section on this later) make it much easier to make use of the buttons without moving my fingers too much, or without changing my grip on the mouse.
2. Clicks
The Basilisk's left and right clicks are very nice, seem solid, but have a tiny bit more resistance to them than the G502. The G502 has much sturdier, more sensitive switches, and I prefer them over the Basilisk, but for this one it mostly comes down to personal preference.
3. Scroll Wheel
Nothing can ever compare to Logitech's incredible scroll wheel feel. The G502 has a snappy, satisfying scroll wheel with a release button on the top, and feels just so nice to use, but I can't discount the Basilisk either. It's scroll wheel is incredibly smooth and tight, and the resistance wheel on the bottom is super nice for getting exactly the resistance you like, a feature which the G502 lacks.
4. Clutch + Buttons
The Basilisk's hot-swappable clutch definitely takes the win here. The metal buttons feel so nice to use, and are a lot less sensitive than the clutch of the G502, making it a lot nicer when using as an alternate button. I personally use my mouse clutch as an active item button when playing League, so having the bit of extra resistance has saved me a lot in the week I've been using this mouse.
Both mice have satisfying, well-placed side buttons, with the Basilisk's being slightly higher up on the mouse and the G502's being a bit lower. The G502 does have 2 extra buttons on the top of the mouse, but I rarely used them as I found them to be awkward and they required me to move my finger away from my left click.
5. Software + Lighting
My version of the G502 has 3 on-board memory profiles, a hot-swappable function which can be bound to any of the mouse buttons, but no RGB, so I'm going to use a screenshot of my G900 and the Logitech software since I'm pretty sure most new G502s have RGB.
Button re-mapping is pretty intuitive, and the 3 onboard mouse profiles are quite nice as well. The profile swap button was in a kind of sucky place by default, and after accidentally pressing it one too many times I just unbound it, and stuck to switching profiles via the Logitech software. The RGB editor is pretty simple, and the RGB on my G900 is pretty and understated, but was usually covered up by my hands
. Overall the Logitech software is dated looking but serves it's purpose, which is nice for people who want to use the software once then leave it forever. 
Like the G502, the Basilisk has on-board memory profiles, and the ability to hot-swap them, however it has 4 profiles versus the G502's 3, and the hot-swapping button is on the bottom of the mouse, and the Basilisk has an LED on the bottom of the mouse to aid in distinguishing each profile. Setting mouse button functions in Synapse is quite easy (this being the first Razer mouse that I've actually really used Synapse with, with my Aetheris I just bound some functions to the side buttons and left it), and the amount of customizability in Synapse is pretty insane. Setting up RGB is pretty easy, as Synapse has a similar simple editor, but the ability to use the Chroma Studio for more advanced effects is awesome. Chroma Studio does have a pretty steep learning curve however, so for those who are new to the software having the option for a more simple editor is great.
Overall, I prefer Synapse 100% to the Logitech Gaming software. Way nicer looking, easier to use for beginners, and (contrary to what many on the forums are saying :p) way more reliable. While using my G502 and G900, I often found that the Logitech software wouldn't open upon me turning on my computer, or would randomly crash or close, and no matter how many times I reinstalled it still happened.
6. Materials
Both mice have a matte finish with high quality feeling materials, however I've found that the Basilisk doesn't seem to like to hang on to dust and "gamer gunk" quite as bad as the G502. The G502's LED is pretty, however the Basilisk has intense, bright LEDs in the scroll wheel which just make it so much nicer to look at imo. Both mice are easy to grip, with rubber on both sides, and are comfortable to use for long periods of time without getting slippery or awkward.
The G502 has tunable weights, however I found that the Basilisk's weight was almost exactly the same as what I had in my G502.
【☆】Conclusion【☆】
Overall, I love both mice. They're comfortable for my smaller hands, feel nice in-game and are very solidly built. In my opinion, the Basilisk is a great alternative to the G502 for those of us with smaller hands, and is definitely a solid mouse overall!! Thanks for reading my review, if you've made it this far, feel free to leave me any questions or feedback!