I think I can give you some perspective. I have a 335i and used to be very involved with Z-cars. I had a 72 240Z with a swapped in L28ET that made just short of 400hp on the dyno. It was a fun project, but I'd never do it again.
The Z32 is a beautiful car. When it was introduced in 1990, there was nothing like it and it was competitive with much more expensive cars from a performance standpoint.
The thing is, the rest of the world has caught up in the last 15-20 years. The performance of a stock 300ZXTT as you would experience it on the street is no longer in a league of it's own. Stock 335i will beat it 0-60 and take it by a about car length in the quarter.
The 300ZXTT is still an attractive choice as, frankly, I love the way it looks. The problem is that you are looking at at least a 17 year old car. If you can find a low miles example that has not been flogged to death and the price is reasonable, it might be a good choice. That combination is very difficult to find, though, and as time goes by, it only becomes more difficult.
I wont sing the praises of the 335 in this post, I just did that in a recent post, so you can give that a read. I will say that you should not underestimate the luxury aspect of the 335. It is not as in-your-face as it is with other cars that brand themselves into the luxury segment, but I can tell you that there is literally no comparison between the 335 and the 300ZX in terms of aesthetics and luxury.
At the end of the day, you can build up any car to be very fast. If 400HP is your goal, it can be done at fairly reasonable cost on both cars. For me, the 335 was the best choice. I've only owned it for a couple of weeks but so far I have not wished anything on this car was more like a 300ZXTT.