[Turbowned]

I would personally take a RWD car over a FWD just about any day, so my opinion might be skewed a little. However, consider this:

The FR-S is brand new. Your Celica is 8 years old. Talking about first year problems while driving around an 8-yr-old car is silly! The first year problems are warrantied; you're on your own if the Celica breaks (not that it should; they're obviously reliable). Toyota Care covers the first 2yr/24,000 miles of maintenance, leaving you with just a couple oil changes, tire rotations and an air filter through the remainder of the 3yr/36,000 miles.

Assuming you have less than 100k mi, you should be able to trade it in for around $7k or sell it privately for a bit more. If it's getting close to 100k, I'd trade it in beforehand to avoid the big value drop when it reaches it. If it's already over 100k mi, you're talking around $6k for the car. Ish. I'm referencing NADA trade-in values here.

Fuel mileage is the same between manual FR-S and manual Celica GT-S. That's a wash.

The FR-S will outperform the Celica, slightly. The Celica has about 250lbs on the FR-S, but the FR-S makes 20hp and 21lb/ft more (referencing Edmunds, I thought the 2ZZ made 190 but they say 180?). The rear-drive layout, low CoG, balance and LSD will ensure that the FR-S can at least out-corner the Celica. 0-60 times are very close. (does your Celica have LSD?)

I guess all I can say is drive the car and see if the FR-S feels that much better to you to justify the car payment. Getting a new car when you have a perfectly good one is an impulsive buy no matter how you slice it. You don't need it, you want it. Find out if it's worth it and report back!