I'd go for the IS300.
Why? It's a whole diffrent class, a much higher class than the MSP.
Yes, it costs more, performance wise, yes it'll kick the crap outta the MSP.
If you're planning to mod, I'd rather mod a RWD car than a FWD, it also already has quite an aftermarket.
Is the price diffrence worth it? That's debatable.
The MSP you get alot for what you pay for.
But all cars are the same. With the IS300, you get RWD, Lexus quality (mazda quality can't touch lexus quality sorry), a more luxerious ride, better seats, defentaly, I've heard too many complaints, and I've seen picture of the MSP's seats being flat...and they are...I thought my seats were bad...heh
You also get more safety features.
Obviously the IS300 is the MUCH better buy. I'd go for that, if you don't mind spending the extra cash.
In the long run, it'll probally last longer, keep a better resale value, and be better to mod.
Also, with FWD cars, if you get too much power, you gotta deal with torque steer. Also I don't know what you want to do...but maybe this won't help, but I'd much rather have a G35 Coupe than an IS300

But then again, the G35 is more expensive than the IS300...
but all and all I'd still go for the IS300...
because...let me ask you this...could you ever possibly drift in an MSP? When you're with a girl, could you whip open the sunroof on a beautiful day?
whoo, the MSP may got a nice sound system...but over the IS300..that's all it could offer, for the price the MSP is good...but the IS300 is just a totally different league.