httpwebwitch - 3:11 pm on Feb 16, 2012 (gmt 0)
Products that can be used with a service? that feels like a subfolder to me, not a subdomain.
It's so subjective.
Don't stress by thinking that it matters very much. Some people will notice that it's a subdomain/subfolder, but most people won't, the majority of your visitors don't care, and lots probably arrived via a bit.ly or goo.gl link so they have no clue what the URL is anyways.
So when you decide, decide for yourself so when you're maintaining the site you have a coherent organization to things and an extensible convention that works.
BTW I don't think it makes much difference to SEO, if any. Other factors are far more important.