I think it depends on how useful you think either is for your long term career. If you think the PhD is not necessary, but the 3 months would give you a leg-up then do that. However, if you don't see yourself getting a job without the PhD then I personally would get the PhD out of the way and *then* look for other roles.
Also, although best intentions may be there to work evenings/weekends on the PhD, once you've worked all week and your gf/bf/family/friends are wanting to see you, or you just want to catch up on some telly, then that may go out the window.