1515 or 1521?

[Sower]

So I've heard a couple of opinions, but I would really like to know why I should or shouldn't go with the 1515 vs the 1521. Can anyone tell me what the big difference is between the two besides the physical size? I've been on the Neu website, but it doesn't tell me the difference in power, efficiency or any of that info.

Do any of you have a little more detailed info on the two motors? Thanks.