Waterfall is a nice green selection, although there are newer clones that are probably improvements on that form (better fall color mainly). It can show some stronger pendulous characteristics than the other two if grown in similar conditions.

There are several forms of Crimson queen and Garnet these days, but the original forms (or at least what I think are the original forms) tend to be a bit more compact and softer in texture when compared to Waterfall. Both CQ and Garnet show a reddish purple coloration in spring that changes to some level of orange bronze red coloration in more sun. Fall colors are both generally fairly bright orangy reds. Most consider CQ to be more sun tolerant, but both get sunburn in my area so I can't say for sure if this is true. Most sources say Garnet is fast growing, but in my experience this is not true. From what I've seen it tends to send out many small branches rather than a few long ones (which is what waterfall does in younger age). Crimson queen seems to grow fairly quick when young but soon starts to show more of these small branches to make it more compact in appearance. Garnet also seems to grow fairly upright (although it still forms a mound).

As I said there are multiple forms and you often see other cultivars sold under these names even from respectable nurseries simply because it is difficult to tell the differences when young, they become more evident with age.

I have 2 AP waterfalls and they're doing pretty well with the Texas hot weather, however every location is unique.
Also I have a Garnet, but if you are willing for green foliage and form I assure you that Waterfall won't disapoint you!

I think that most (if not all) green dissectums are not very sun tolerant. My Viridis was originally planted in a sunny location and I moved it after the first season because the leaves burned up so much. As a general rule, most red JMs can handle more sun than green varieties.

Garnet and Crimson Queen can handle a lot more sun. My Garnet has grown very fast compared to CQ -- it is taller after 3 years than my CQ is after nearly 20 years. Garnet needs a lot of sun to keep its red coloration. Mine is in a fairly shady location with just a couple hours of direct sun daily, and it changes from red to green by mid-June. My CQ keeps its red color all summer, but it gets more sun than my Garnet as well. I've also got a Red Dragon, and it holds its red coloration better than any others.