I'm also in a similar situation. The name I'd like for my webapp is taken and i'm considering hq.com or app.com. Or would it be better to use a .net or .biz?
My personal opinion is to pick a more unique name. But, I'd say postfix is better. The domain name without the domain code is more brand-able than the full domain e.g. BaseCamphq is recognizably different, but doing basecamp.net wouldn't be as unique.