I've had great luck with both of my '91s, however I have found that '92 parts fit better )o2 sensor, plugs, etc.) so I tell the parts house it's a '92. As far as the A/C, I can't imagine any 10 year old vehicle that doesn't need to be completey changed anyway. I converted my '91 and it'll freeze ya out; even better than stock.
The '94 have the better heads on the motor. Ford changed the casting to avoid cracks.
My '91 was kinda rare stock. XLT with all power interior, manual trans, manual t/c, and manual hubs. I just wish I had the leather interior and sunroof.