They do both let you steal UTs, but their methods are completely different, so you could put both on the same ship and use both abilities in full.
Tobias is more convenient to use because you can grab something just by sailing past another ship, even a friendly, but you can only take one at a time, and removal/unloading restrictions still apply.
Rollando is much more powerful because you can grab as many as you want, even UTs that normally can’t be taken (Nemo’s Plans!), plus you reveal non-uniques. The downside is that you do have to make contact with the other ship, which takes some careful maneuvering if you don’t want to ram, board, or be pinned.
ADDED: Another subtle difference is that with Tobias, only you get to see which UTs the player has, and only the two of you will know what specifically you took if it was facedown. With Rollando, everything on the ship gets flipped face up so other players will see it too, and everyone will know what you took.