It's whatever you prefer, although you'll come to love whatever you buy.
For me:
I love the 34 headlights over the Supra
I love the Supra's rear guards and wide low stance
34's look better with spoilers, Soops look better without
I loooove the hood on the Supra, its similar to a Viper
Interior R34's look better, but a bit plastic
The Supra tachos are hideous
Kitted 34s look way better, Supras can get really "ricey"
Boot and rear seats on the Supra are non existant (MUCH worse than the R34)
R34 is going to be newer, but then again the 2J is an amazing engine
They both sound good
NA Supras go faster (mine keeps up with stock Evos and destroys stock GTSTs and Silvias)
I should stop heh.
I dunno, I love both. Based on looks alone I take a Supra over an R34 anyday, unless its GTR. Now that's an menacing looking car.