I use deep frames, because I wanted to be sure of having enough room for colony buildup in a hive 4' long. That was about the maximum hive length that I could easily transport by myself when empty to set them up as I add to my yards. Longer ones are heavier and more awkward if you are not tall. :)
It takes 33 frames and is basically equivalent in volume to a 3-deep Lang. I can overwinter (or start from split) two hives (one in each end of the box) and add new hives as needed. A strong hive will grow to fill the entire space by June, at which point I am harvesting a few frames of honey most inspections. I have been splitting aggressively this year, so most of my hives are smallish (5-7 frames of brood, at least the same of stores, 1-3 of pollen) but have one last large round of brood being laid before shutdown in November.
With medium frames, you might need to be somewhere where a smaller colony would still do well. If you are way up north and ordinarily need 4-6 medium boxes of stores to winter....hard to fit in a long. You could super the long hive in fall but that defeats the point of not lifting boxes a little bit. I am not dogmatic though. One thing I love about the long hive is how flexible it is in terms of how much space you have and when.
(For example I am going to try a two-queen system next year for fun with a colony in each end of the long hive working a common stack of supers in the middle. Will see if it works.)
My frames are foundationless, and I have never had any trouble with collapses in the hive (even with open screened bottoms and daytime temps in excess of 102F). Sometimes honeycomb has collapsed when I am bringing deep frames home in the car for harvest. It can take several months for the bees to attach comb on all sides in a deep frame, but it is plenty strong nonetheless.