Both are great. Tom and Jennifer are the best from Nuance, and they can be used with any program. They're much easier to understand, but they don't sound as natural as Acapela, and they don't come with a dictionary editor either. So, unless you're using a program that has it's own editor (Like TA and many others), you're kind of stuck with what you got.
Acapela sounds more natural, but harder to understand at times. I find Heather the easiest on the ears. The drawback to Acapela is that they can only be used with the program they're sold with (this is Acapela's licensing terms). In this case, if you get them from NextUp, the voices will only work with TextAloud - no other program. They come with their own dictionary editor, which is a bonus, but it's not really necessary because TA already has an excellent editor.