I've been wrestling with the same problem for months man, I swear I've switched back and for so many times I cant even count now. This weekend though, I think I'm going to pick up my s13, and I'll tell you why:
1. the s13 is straight up cheaper. I get as much enjoyment out of working on a car as I do driving it (well, close

), and I feel like if I spend half as much on the car to begin with, I'll have that much extra money to do whatever I want with it down the road. that was really worth something to me. (*yeah I know you can find $3000-$3500 s14s, but they've ALWAYS got somethin wrong that you shouldnt have to deal with)
2. s14s dont necessairly have lower miles. the s13 I might pick up is a 91 with only 90,000 miles on it, and I've seen lots of other s13s with even less. If you're looking at a 95 or even 96 s14, you can pretty much expect around 100,000 miles unless you want to pay 6 or 7k for the car (which I couldnt).
3. I LOVE the look of the S14s, but I think that a freshly painted glossy black s13 hatch with kouki (180sx) tails and something mean under the hood is pretty tough competition as well. I'd rather be able to build the car I want rather than have something out of the box, ya know?
4. I dont know why, but I just liked DRIVING the s13 better. I dont know what it was, maybe someone else can contest to this as well, but I felt much more in control of the car, like I was a part of it instead of just driving it. The s14 felt more bulky to me, and the seeming lower seats (are they really? I donno..) made it less of an engaging-type ride. I donno, these are really my opinions about how the cars feel... you should drive both and make your own conclusions.
Hope this helps man!