which cab should I add?

It's going to be awhile 'cause we still don't have a full band as of yet but when we do and we start playin' gigs and stuff which cab do you think would be better?

Right now I'm using a GK 2x10 GLX

I'm looking to add either a GK 1x15 or 4x10. I like the look of the 4x10 better but as far as tone and range and what not which would you suggest?

Thanks!