Interface or USB Mic?

[ParisStarlight]

I am looking to record some demo material of song ideas at home. I have a small room (used to be a coal room) that I will use and the acoustics are quite nice in there. My question is am I better off buying an interface or USB mic?
I am looking at either the Scarlett Solo or a Blue microphone. In the price range I have I can purchase a Snowball or Yeti mic. Leaning towards the Yeti if I go that route. If I purchase an interface I currently have a Senheiser 609, EV PL6, and a Shure SM58 I could use with it. Am I better off using the cheaper USB Blue condenser mic or using my existing microphones with the interface?

[Jusca]

I'd say interface for greater flexibility to record 1+ mics at the same time, future upgrades, and greater flexibility when recording.

USB Mics if you absolutely know you won't record anything else in the foreseeable future from sources like an acoustic pickup, electric guitars, etc.

[Psalad]

I agree, for portability the USB mic can't be beat. But if you're looking for the long term, a USB mic becomes unusable once you buy a better microphone. So for me, it would be interface.

[ParisStarlight]

I am leaning towards the mic. Main reasons are that I am not too worried about doing "serious" recording by myself and that none of my instruments have pickups in them.
Main thought would be is the Blue mic going to sound better quality than the mics I currently have? If not then I see the interface as the obvious decision.

[MikeBmusic]

A room sound that is pleasing to your ears when playing may not record the same way. A condenser mic will typically pick up more of the 'room sound' than a dynamic mic. Another vote for an interface ) get one that has at least 2 mic preamps so that you can go to a 2-mic method without having to upgrade again.