1) What frob and Tom said : work out your requirements first! You need to do some proper project planning...
2) From your planning you should realise that most projects naturally break into phases (e.g. concept, pre-production, production, marketing, debug, ship, post-launch).
3) You'll probably need both artists, but not for the entire project - so work out what phases you need each for and contract accordingly. It's likely you'll need each multiple times - for example the concept artist can help prepare your marketing materials.
4) If you have a genuine tie-break situation (e.g. two 3D artists who are equally as strong on paper and at interview), go for the one who's personality will best fit with the personality of the team (the whole team - even a team of remote workers),