For your reference, we have made a special table that allows you to sort by relative price, my rating and rack thickness.
The highest rated is the 3U workhorse (303) because its great value, good form factor, price and quality. The lowest rated is the thin 2U 302b which has heat problems, but is still a very popular seller. Most of the newer units (310b) and (333B) get low rates not because of quality but because of their much higher cost relative to what you get. The highest rated unit that is expandable (XL) is the 315.
See the whole table here
