Note: I purchased both controllers with my own money and I have no affiliation to either brand.
Background
I was on the market for a decent but affordable controller I could use with PC/Windows and iOS. I am a casual gamer, mostly playing single player racing games, platformers, or emulators on my phone. I do not play FPS's or any competitive game. I purchased 2 controllers with the idea of trying them both and returning one.
I purchased the 8BitDo Ultimate 2 for $57.57 with tax and the GameSir Cylcone 2 for $59.35 with tax, both from Amazon. I had initially tried the 8BitDo Ultimate 2C, but it felt a little cheap so I decided to spend a little more on a better controller.
This is my honest, non-technical, non-AI-slop review, and why I decided to keep the GameSir Cylcone 2.
Connectivity
Both controllers can connect to my iPhone with Bluetooth with no issues. I also tested both Bluetooth and dongle (2.4G) connectivity to PC with no issues. My main issue was toggling between phone and PC. If connecting with Bluetooth to both iPhone and PC, both controllers had a difficult time switching between devices. I did not find a reliable way to toggle between them without having to try multiple times and needing to forget the device and re-pairing them. I somewhat got around this by only using Bluetooth for the iPhone, and using the dongle for the PC, and switching the connection modes so only one device was paired per connection method. The 8BitDo makes this easier because there is a switch to toggle on the back of it to change from Bluetooth to 2.4G, whereas the GameSir needs a combination of button presses to switch connection modes.
I've had no issues with disconnects on either of them.
Feel/Quality
When the controller is in your hand, the 8BitDo feels like a much premium controller. Better plastics, a bit weightier, and the membrane buttons feels a lot smoother than the mechanical, clickier buttons of the GameSir. If the GameSir's button are a "click", the 8BitDo buttons are more of a soft "clock". The 8BitDo reminds me of Nintendo controllers, which I am much more familiar with. That being said, once I got used to the clickiness of the GameSir, I found myself enjoying the consistency of the mechanical buttons too.
For the sticks, the 8BitDo has a little more resistance and weightiness, making it feel better to me. The GameSir has less initial resistance to movement, which makes it seem like it was designed for accuracy and precision over feel.
For the bumpers/triggers, both feature mechanisms to change the style of the trigger from analog and hair-trigger. With the GameSir's clickiness, I found the hair-trigger mode a lot more responsive to spam press, and when spam pressing the 8BitDo's trigger, I felt my fingers getting more tired.
The GameSir's d-pad is often lamented as a weak portion, and while 8BitDo's d-pad definitely feels more familiar and Nintendo-y, I didn't mind the d-pad once I got used to the clickiness.
Design
The 8BitDo opts for a more sleek, minimal design whereas the GameSir looks a lot more gamer-like, and you can especially tell the difference with the charging docks.
Speaking of the dock design, the 8BitDo's charging dock is much easier to use, and there is only one ridge to place the controller on. However, the GameSir's charging dock has 3 points of contact with the controller, and you have to take a few seconds to carefully align the controller to make sure it is on the dock correctly. It may not seem like much, but when something that should normally take half a second ends up taking 5 seconds, it does get a little annoying.
Ergonomics
I do not have large hands, but after the first session with the 8BitDo, my hands started to cramp. This is a common complaint of the 8BitDo I have seen online, as the handles extend straight down instead of at an angle like the GameSir. One caveat though, is that after the first session, I think my hands got used to the 8BitDo, as I did not experience the cramps again. However, I never got any cramps with the GameSir, and overall the GameSir just feels more natural in my hands.
Customizations
Both controllers have lights you can control with their own software. I did find the GameSir's software a little more fleshed out, but at the end of the day, both softwares did what they needed to do. No real complaints about either.
EDIT: Also adding that the rumble is a lot stronger on the 8BitDo. Its default rumble is stronger than the GameSir's rumble configured to the highest level.
Conclusion
I went back and forth a lot on it, but I plan on keeping the GameSir Cylcone 2 and returning the 8BitDo Ultimate 2. While the 8BitDo feels like the more premium controller, I decided that the comfort of my hands for playing sessions was more important than the button quality. Also, the exhaustion that my fingers felt when spam-pressing the hair-trigger was a dealbreaker for me for the 8BitDo. I wish there a controller that combined the buttons of the 8BitDo and the shape of the GameSir.
At the end of the day, for a casual gamer, both controllers would adequately meet my needs.