You are certainly approaching the hobby very sensibly. This sort of question rarely comes up because it is something decided upon sort of on the spur of the moment or by the means of what's available.
Saltwater fishes are a wide variety of creatures that have differing needs. As you've learned in the FW hobby, some fish have needs for tall tanks. In the SW hobby there are also fish with different needs.
To answer your question, you want to decide on the types of fishes that you are attracted to. You have indicated a small Maroon Anemonefish. It may be small when you get it, but it can grow to 6 inches long if it is a female. This is an example of, how in the SW hobby you have to plan for allowing enough room for the fish when it grows. This fish will stake out an area of the landscape. In the wild they usually stake claim to about 8 to 10 cubic feet. I usually recommend them for a 45 gallon or larger aquarium.
Some examples: If you are interested in fish such a Gobies, they often stake out substrate space and reside in or on the substrate. Butterflyfishes will tend to cover all heights where there is rock, with no particular preference. Dwarf Angelfishes tend to remain low, staking out an area in the landscaping. Anthias, Damsels, etc. will often remain in the water column.
Marine fishes are used to traveling, unlike many of the FW fishes. In general, the longer tank is preferred UNLESS you want fishes that need substrate space. SW fishes don't swim up and down, so longer and shallow is generally preferred for most reef fishes.