the two abilities are slightly different (see MM p305-306).
alternate form is like polymorph, with a limited number of forms. change shape allows you to assume any from, though often within the limitation of a specific type of creature.
i compared the qualities of each ability, and the only difference i really notice is that with alternate form you lose the natural armor and Str, Dex, and Con of your standard form, and gain the ones of your new form. otherwise, there is very little difference.
i think alternate form sounds the best for that reason, but i want it to be able to take any humanoid or monstrous humanoid form (human one round, sahuagin the next, then a lizardfolk, and then a dwarf).