Its a matter of opinion - I think tough/smart or charming/smart are the best combos, since you will need smart skills as well as some interview skills, and this gets you discounts for both. But - in time - you'll get all the skills, so it won't make much difference in the long run.
The only combo I don't see the point of is tough and charming, since you will normally play only one or the other, not switch. But other folks may have better ideas on that.
What I don't know is whether being totally tough gives you a slight edge over smart/tough (with same test equipment) in interogating/intimidating. If so, it may be a good thing in the long run.