I have both detectors mentioned. In fact they are the two detectors I pretty much always use. I also have a Tesoro Compadre, but I use it mainly as a loaner for friends, family and permission granting landowners who hunt with me.
I've had the F5 for close to two years and the F75 LTD since last fall. I've made a lot of excellent finds with both and they are my favorites of the detectors I've owned to this point.
Comparing the F75 to the F5 isn't really a fair comparison. The F5 has a MAP of $499. The F75 Special Edition or Black or whatever you want to call it (I think FT should have called it something besides "special edition" since that's the way the original 1st gen. F75 was described on the box) has a much higher MAP of $1249 with an extra coil included.
The F75 absolutely has more depth. Nasa Tom Dankowski says 1 1/2 to 2 inches more depth on coin sized objects in the ground and my experience tells me that's about right. With the 5" coil on both detectors, I think the difference might be even more pronounced. The F75 5" is an amazingly powerful little coil.
In other areas of performance - target separation, recovery speed and numeric resolution, both units are very similar.
The depth advantage of the F75 is indeed a big advantage, but I wouldn't agree that the F75 is light years ahead of the F5.
The F75 has several features I prefer over the F5, the biggest being the trigger pinpoint which I think is far superior (and apparently more durable) compared to the pushpad pinpoint on the F5. Brian mentioned other features where the F75 has the advantage also including backlight, more durable battery holder and more available aftermarket accessories. Ergonomically, the F75 with the 11" coil is better balanced than the F5 with the 11" coil.
The F5 is a smoother running detector that loves high conductivity coins. I can't fully explain it, but I prefer the overall audio experience of the F5. Besides the independent threshold already mentioned, the F5 has the advantage of giving a continuous ground balance readout. With the exception of the pinpoint, I like the knob-driven interface of the F5 better than the more menu-driven interface of the F75.
In older areas where the chances of deep coins and relics are good, I'll take the F75 every time. In areas where the extra depth is not as important, like postwar yards and tot lots, the F5 more than holds its own.