Flame offers more out-of-the-box features, while with Bullseye2D you have to build many things yourself, but it doesn't force you into a predefined structure as much. The API is more prodcedural style, making it relatively quick to learen in its entirety, without too much "magic" happening under the hood.

I think for most people Flame would be the better choice (it is also been around longer, which means better support...), however, I really love extremly simple approaches. I also believe it has advantages when you tailor your systems and architecture to your own requirements. Lots of stuff in 2D Game Development is also extremly simple (so often having to deal with a complex physics module or rendering pipeline or entity component system is contraproductive).