OP mentioned wanting to break 600awhp...what's your budget? When I see numbers thrown out I always wonder why that number specifially?. Personally I'd rather have a 400whp car with instant spool at lower rpms than a 600whp beast that has an extremely short and mostly unusable powerband.
If you've got $20k to drop into a car then you can get there with either car. But IMO it would be a lot cheaper and easier to do it in an Evo. But you may prefer the STI interior and cockpit (I know I do) over the Evos.
I used to have a 2.5L bugeye and Evo 8 but am currently down to just my Evo. With just a ported Evo 9 turbo and supporting mods it's just shy of 400whp....on E85. On 91 octane it's in the lower 300's. The performance mods including tune cost under $2k. That's a bargain to say the least.
Swapping in a BBK Full or FP Red or Black and upping boost a lil would put me closer to 500whp (on E85). But those are stock-ish frame turbos that like high boost. Just going to a GT30R or 35R on E85 would adds the power you want but I certainly wouldn't feel comfortable running those on stock rods at that level. If the motor was built then you'd be gtg.
400whp in a 3200lb car (Evo 8/9) feels really great. I can't imagine what 600whp would be like! But whatever car you go with and whatever mod path you do take...plan it out in advance. On my previous Evo 8 I tried out a bunch of turbos and things before finding a setup I liked....wasted a ton of money.