I'll put in my .2 cents. And remember this is just an opinion. I wanted a LS1 Z a while back..
I'm a GM Certified Tech. Mr. Goodwrench.
Stock for Stock a Z is faster.
They are immensly hard to work on and expensive to work on/modify.
To me, the build quality is unbelievably cheap.
They suffer from a list of electrical problems (many people fight me on this until they look up the TSB's or work on them themselves... the 2000 models seem to be the worst for this).
If you build serious power your going to be looking into a rearend.
They leak oil. I've repaired a few SS cars with major oil leaks around 30-50000 miles. But that's a GM thing. They all leak oil from somewhere. My 100K 5.0 has no leaks. And almost all of the 100K 4.6's leak no oil.
They ALL have a pinion seal leak from the rearend eventually. This can be cured, possibly, by an aluminum driveshaft. But if not, you can fix it and fix it.. it will leak again.
I wanted one until I drove them around and worked on them all the time. I'll stay with my stang. I love Camaro's and GTO's and especially Vette's. But if you dont' have a ton of cash... you may want to stick with Mustang.
Just an opinion.