Went through this very dilema a few years ago. (When the G35 Coupe came out, and I just about sold the Supra and bought one...)
Well, by the time I really sat down and looked at what I like, and did not like about the G35 v/s this old supra, I decided to keep it, and do some mods that I wanted to do.
The cold hard fact is that any "stock" car short of a 100,000+ sports car will need some mods, and even those cars get modded by the owners. (Look at all the shops who specialize in putting turbo's on exotics..)
Big brakes are expensive, so are turbo upgrades, fuel computers and all the other "mods" we do to our cars.
I really like what you have done with the alcantera on the trim panels. I've been wanting to do my entire dash and all the trim panels for years. (Perhaps leather on the dash, and alcantera pillar trim, headliner and other stuff to match and accent the leather seats I'm going to do one of these days.)
That is just it. "One of these days" adds up to many thousands of dollars spent and never recovered, but there is one benefit to starting out with a car you own... YOU don't have to pay for it up front, and then lose tons of money on the depreciation of the car on top of knowing your never going to get back what you spend on mods.
Example. 2006 G35 6MT with the wing is about 38k new. In a few years it will be worth 10k eventually, and when it's as old as my 88 Supra Turbo, it will be worth a similar value I'm sure. Let's just focus on the 28,000.00 in value it will lose in the first 10 years or so. (Perhaps sooner than 10 years.) That is some serious mod money! Just think of all the cool things you could do with that 28,000.00 and then keep your Supra.
Your choice of course, but a very nice GT coupe is a very nice GT coupe. (Be it the G35, or the MK3 Supra.)
One more thing. Your going to see nice G35's all over the place for the next few years, but how often do you see a really nice MK3? Not likely as most are ratted out, and look like crap. The nice ones are few and far between, so you are not going to see "yourself" at the stop light anytime soon.
Just my opinion, but I've been where your at.