Depends on the car . . . assuming it's a Celica I would opt for show over go. Even if you put all your money into performance on a Celica there are stock cars like the Evo, WRX STi, 350z, Rx-8 etc. that Celis just can't compete with. Given that, why sink money into a car that will get toasted by better equipped stock vehicles? What you CAN do is look as good or better than all of these cars though.
It's all about picking battles you can win!
Well, no matter what you do to your car, there will always be a faster one. Don't listen to this comparison because these are cars that are not in our class.
As for your question though, it is all preference. If you want turbo and are real serious about speed mods, then do that. Or if you are more looks, go for that.
Me, I'm both. I want my car to look good, but don't want to be a bitch on the road...know what I'm sayin? I know that I will get beat by many cars, but I will also be able to beat many cars. What I did, is bought the expensive performance stuff first (bolt ons) and bought cheap appearance stuff. Now I am spending money on the expensive show stuff. You could do it that way, or the reverse way. This may be confusing when you read this, I hope you understand.
But now I am back to performance again, still debating on everything.
Hope that helps. Good luck and keep us updated.