(This is a bit late-so my apologies if you've already made a decision)
First off--congrats on multiple offers! (i hope you haven't held on to two for too long though!) but I think you're actually in a really great position still being undergrad. What I mean by this is I don't think you're quite in a dire straits situation yet in which you should compromise on your research fit. Sure, if you've been out of school for quite some time and you have applied multiple rounds, then maybe I would be more inclined to take one of the offers. But I think you really have so much time to get some work experience with this PI whose interests are perfectly aligned with yours. While you can never guarantee the future, I would imagine that you would still be incredibly competitive in years to come, as you already have been successful in receiving two offers while still in undergrad! You will only strengthen your application with added work experience! I also don't think theres anything wrong with staying at the same institution (particularly if you like it and they like you!)
Either way, congrats again..and best of luck!