I think you'd be fine with either or, and at the end of the day they're two rather different experiences, but Generations is probably the safer choice despite Adventure DX's novelties. Generations is much more to-the-point, with minimal plot and only two gameplay styles that share a lot of similarities, a more direct method of level and mission selection, and again, it's by far the more technically sound game of the two. Still fun, but also kind of jank at points. Adventure DX is slower-paced (that is, compared to Generations' hyper speeds), more story-oriented and has six playable characters each with their own distinct gameplay styles, plus a virtual pet side mode that people go gaga over, although it's old which means occasional glitches, awkward voice acting/cutscenes, and a lot of experimentation given that it's the first 3D Sonic game. I'd choose between Generations and Adventure DX, depending on what you like most in your game. Lost World is okay but lacks identity and consistency, which makes a lot of it kind of wasted potential. Adventure DX on the other hand has its own interesting novelties that Generations doesn't have, but it's also more archaic given that it's a 20-year-old game. Generations by far is the most polished game of the four, while Forces in comparison is just a pale imitation of it in terms of gameplay and level design.
