Difficult (and probably a bit unfair) to compare - and I must emphasise that I am no 'driving god' so I can't really comment on technical stuff.
But generally, I feel that the MR2 handles better. It's lighter obviously, which helps. It feels slightly more 'planted' than the Celica - I feel more confident throwing it into corners (not that I do a great deal of that at my age but just enough to have some fun!). It definitely feels like a sports car, with the low driving position, and the engine behind you. But the Celica is also fairly low compared to many modern cars. I've got used to that now.
I found that the handling of the Celica improved when I changed the 18" wheels for 17" (the barely used tyres on the new set probably also helped). But I still find that the Celica 'tramlines' on rough surfaces, which the MR2 doesn't. I do find this a bit disconcerting, but am getting more used to it. The suspension has also been lowered, which I know a lot of people do without problems, but my OH has his doubts about this. Having said that, he's a bit of a stickler for 'keeping things stock', so he prefers the MR2 because it's basically as it left the factory - with the exception of refurbished brakes and pre-cats removed.
Acceleration-wise, I guess the MR2 is quicker because it's lighter, but there's plenty enough power in the Celica for my purposes. I did consider a T-Sport (and we test drove one) but the advantages of lift didn't outweigh the higher initial cost, insurance and road tax (not as a second car, anyway). I don't 'need' lift - I just need a car that is quick enough to overtake when I need it, can cruise happily on the motorway in 6th (I do about 250-300 motorway miles a week), is reasonably fuel efficient and is nice to look at Both the MR2 and the Celica fit those requirements.
I didn't really plan to get a Celica as well when I got the MR2 - it was going to be my daily driver. But I've always liked the Gen 7s, and this one came up locally, and the rest is history :rolleyes:
Overall, I like them both, for different reasons. I actually find that I get more attention in the Celica than the MR2 - probably because it looks so different to other cars, whereas the MR2 is probably sometimes mistaken for an MX5 by those who don't know
There is a good MR2ROC Forum if you haven't already found it: http://www.mr2roc.org/phpBB3/index.php