This may be the toughest choice for me so far. We have a pretty decent idea on how powerful Mayuri is. And I agree with TN's observations that he is as powerful as he needs to be to further the plot. He was well matched against Gantz, but I wonder how he would do against a melee type, say Nniorta where he isn't matching wit-for-wit. Lisa was - emphasis placed on the past tense - a 2nd seat, and of all the seated positions in the SS, the second seet seems to have the most diversity - some are just about at captian class and some would struggle, as has been shown, with the 4th (labled 5th) seat of another squad. Now, just thinking out loud here, we know reatively little about Lisa althought he feeling I get is that she would be much more powerful than, say, an average 2nd seet, by mere fact of the vizording of her power.
So, anyway, I'm voting for Lisa on this one, as I see Mayuri as a relatively narrow nitche character, albeit very well matched in that nitche. Given what the potential of Lisa could be, I am thinking...without knowing, for you linguists out there...that she could be not just stronger than Mayuri, but much stronger. As long as she doesn't eat Nemu before the combat. And the chances of that are about as slim as me following though on my new year's resolution.