I hope you're not going to base your decision on somebody else's opinion. The best thing you can do is go drive each car you're considering...then go from there. You're buying a street car not a race car so power shouldn't be nearly as important as the car itself. Each one of those cars could achieve 450+rwhp with relative ease anyway...the only difference will be how you go about doing it.
About the quality, I don't care what anybody says...Mustangs and F-bodies are about as equal as you can get. There's a pretty decent chance that either one could have some problems. My personal experience: I never had a major problem with my '96 Z, each window motor was replaced once but the drivers side was 3 y/o and the passenger side was over 5 y/o when they finally gave up. Also, I had to replace the opti but you wouldn't have to worry about that on a 98+. My '02 is absolutely problem-free after almost 2 years and 26k miles. A good friend of mine has owned three Mustangs ('95 GT, '96 GT and a '99 Cobra). The only problem I can remember him having was cracking the stock manifold on his '96, that's it.
If you really want an opinion, here's mine:
My first choice would probably be a Mach 1 (if it's new; or used w/ really low miles). You can't beat a new/almost new car and Mach 1s do look pretty bada**. On average, they may be just a tad (1-3 MPH in the 1/4) slower than an LS1 (stock vs. stock)...but not nearly enough to be a factor in your decision (at least it shouldn't be).
Second would be the WS.6. It's almost identical to my car (I love my car) but the interior is a little "nicer," it handles a little better and it looks absolutely gorgeous. The T56 is great but the 10-bolt rear is a POS. The 4L60E is nice for an auto (if you go that route). T-tops are nice, I didn't opt for them on my '02 but sometimes I wish I would've.
Third would be the Cobra. I really liked my friend's '99, handled good, pretty quick (this was post-"fix"), looked good, interior was pretty nice...not much about the car I didn't like. The only thing that pops into my head was the shifter location...too far up for me. I think I almost had to lean forward to get it into or out of 1-3-5, I could've lived with it though. I guess if you're leaning towards getting an auto, a Cobra would pretty much be out of the equation.
Your opinion is the one that ultimately matters though...so do yourself a favor and go drive the cars. After you decide which car you like the most (that's the car you should buy), then look into getting more ponies from it.
Good luck with your choice.
