The 2.3L motor is a little rougher around the edges but with a simple pulley you can put in yourself and an air filter you can get close to 200HP at the wheels. The 1.8L motor is smoother, gets a few mpg more but realistically it's about 160HP at the rear wheels, if that. A pulley will cost you 2-3 times as much and is unproven and and may get you 10HP, add a chip for another 10HP, still leaves you quite short of the 2002.
So if you are never going to mod it, I would go for the 2003 and have the smoother motor and better mpg. If you want to mod it, go for the 2002 and beat out some european cars costing much more (I beat out a older Acura NSX) with mine.
One last thought, if it were me, I would get the C320 coupe that came out 2003. Smoother engine, quick off the line and can also be modified cheaply with an underdrive pulley kit that gets you another 12HP in the midrange. Also, for a mere $10K, it can be modified for another 100HP.
I switched from my 2002 coupe to a 2004 sedan purely because we wanted a sedan, not because I didn't love my 2002.