I would say get gears, cuz I just got mine put in today (got to update the sig) but I can't drive it hard yet untill they are broken in so I can't comment on that quite yet. I got 3.73's put in and they seem to be very streetable though I can't take it past 55mph yet. I don't rev it any higher than I used to, it actually makes me drive slower
...so far
If I had to choose a mod other than the gears, I would go with ...anything you can get your hands on(except throttle body, probably last of the minor mods)
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1993 Mustang GT - March Ram Air & Pullies, K&N, C&L 73mm MAF, Super Competition Hooker Headers, 2 1/2 in. mandrel bent cat back, High Flow H-Pipe, Pro 5.0 Shifter, 16* Timing Advance