I just tried both Soundboxing and Audioshield. For me personally Soundboxing is superiour because I feel like Audioshields audio is not that great and the hits feel out of sync with the music. Soundboxing is user created and thus far always syncs fine (as long as the user has a good feel for rhytm.)
Yeah, they both have pros and cons.
Soundboxing
+ Tailor made beat maps
+ Well made beat maps are more fun than Audioshield at it's best imo
+ Making beat maps is surprisingly fun
+ Dev updates a lot
- User generated beats means not every song can be played unless you want to make it yourself, and some beat maps are terribad
- Discoverability is hard and the new songs tab is flooded with anime music
- Limited to music on Youtube
Audioshield
+ Can use every song in your library plus Youtube
+ Multiple difficulties
+ Well generated beats can feel like zen
- Beat generation is hit or miss and there's nothing you can do to improve the misses
- Youtube videos took forever to load for me the first time
- Game feels abandoned by dev
As a rhythm game fanatic, I really like both though I prefer a well made Soundboxing beat map over the best generated Audioshield stuff I've played.