Short answer: They are mostly the same. And you can use any service and product with any name.
Longer answer: @aol.com have been used before for our ISP/Access customers, whereas the @aim.com was used for our free users. Now all products AOL is offering are free (except access) and it doesn't really matter anymore.
If you have an @aim.com address you can in fact also receive mail under @aol.com - and vice versa.
In addition you don't even have to use @aim.com or @aol.com at all (since those domains are really crowded and it's hard to get a cool name). You can sign up for other (free) domains at http://www.tunome.com - and can use that email address to access all AOL and AIM products for free as well.
I would actually suggest you either get an @aim.com or a Tunome address, since those account types will get a true internet/inbox mailbox. Which means mail stored there never expires and you can truly enjoy the unlimited storage we are offering for all our users.
