I have owned two 34 gtt's one auto and one manual. The auto needed to be followed around by a petrol tanker as it was only getting around 380km to a tank around town but the manual was doing 480+km around town and 570-580 on the highway. The manual i had was in showroom condition and i sold it for $24,500 at the time. I would not pay more than that and there were a lot of people struggling to sell at that price because there are a lot of poor examples out there. Mine sold quick and when inspected the mechanic was impressed even though the car had only done 68,000kms. Overall it was a great daily driver and very comfortable.
I am now on my second R33 GTR and the build quality shit's all over the 34 GTT. Everything about the cars interior, driveline and exterior(debatable due to preference) is a cut above the 34 GTT. You will be able to find good R33 GTR's for under 30k without to much trouble in fairly standard condition, my fist one was mint and i only ever sold it because i had nowhere to keep it due to work. The interior was like new, the engine rebuilt and everything in perfect condition and i bought and sold it for around $28,000 and that was a series 2 96 model but i know i sold it oo cheap because i had offers for over 30 after i sold it. Overall, an awesome driving experience but a little less frienly as a daily driver than the GTT but you get a LOT more bang for your buck with the GTR.
I have driven S15's and they are a fun driving experience but the GTT and GTR i found better on the street and more practical size wise as well. The S15 is a small car and feels that way inside. I am quite broad shouldered and it feels like a matchbox car to me.
If your fuel bill is not an issue then go a GTR, if you need a more practical daily driver then its the GTT and if you want something a little lighter in weight, on fuel and dont mind a little less room(dont have to carry to many passengers) and are not too tall then go the S15.