One thing I think you might have over looked on your selection is Bombina Orientalis (Fire belly Toad, most lfs mix breeds) They are much more active then dart frogs, much easier to care for (within regards to locating suitable food and keeping your tank stable) and considerably cheaper then darts (on average darts run between 40-200+) FBT tend to run about 4 dollars apiece.
firebellies use absolutely no vertical space IME.
they're pretty much aquatic, spending much of there time in the water, and are not as active as dart frogs at all.
i have both.
the FBT's are impossible to keep fed, as they eat like a trash compacter.
they take Flightless fruit flies, crickets, mealies, etc.
i feed mine about 300 ff every week, and a dozen crickets a week, along with any house flies i may find(my iguana room attracts them).
FBT's ussually go for about 6-7$ a peice in lfs.
good begginer PDF's go for 20-60$, depending on age,species,breeder,and lineage.
Once you get used to cuturing fruitflies, (takes about 2 minutes every weekend) it's very easy to feed PDF's. they eat about 30 or so ff's a peice.
The one thing no one has mentioned about the darts is, opposite of many species, they are in fact more difficult to care for in bigger tanks. Now, I dont think you'd have too much of a problem with bigger species like D. leucomelas, D. tinctorius, D. Azeureus (sp?), or D. auratus. But your smaller thumbnails would definately be out of the question in 50 gallons, although they wouldn't utilize the floor space anyways.
I've found this, through research, to be completely false.
bigger species do better in bigger tanks,yes.
but, it's been proven that thumbnails and Pumillio do VERY well in large inclosures, often being extremely bold.
Alot of people don't even use 10 gallon tanks anymore for thumbs because they feel the frogs are cramp'd.
the thing to remember about thumns and pdfs in general, is do your research.
you'll have to locate any big,medium, and small cracks and gaps and seal them or screen them off, for darts and ff's can get out of small openings.
If you look into it and get some good basic knowledge, a small group of begginer thumbs such as Imitators or ventramaculatus would be awesome.
One thing about dart frogs, they're fairly easy to breed, and most thumbs do most,if not all, of the work themselves.
imagine looking around in your tank, when all of a sudden, out of a bromeliad, a tiny froglet hops out into the open!