If your worried about resale value I'd go the s15 as not many are imported unlike the r34, which will become almost as common as r33's in a few years time (not quite that common but you get my point). I personally think the r34 looks heaps better all round especially the interior as the s15's have those dickey air vents.
My r33 hardly uses any petrol when I'm not boosting it. Both the current sr and rb are very efficient at burning fuel, the only factor influencing fuel consumption is the weight difference, as the r34 has more weight to get up to speed. I think you find both cars will give you similar mileage; you may even get better mileage with the r34 on the highway.
If you’re tall like me go the 34, so much space, comparable to that of a commodore. But it’s all preference I guess. I think you’ll be happy whatever way you go, as they are both great cars, reliable, stylish and more than anything Nissan which instantly makes them good haha, but seriously go the 34.