Longboarding Resources & Q/AGet or give friendly, patient answers to, "Which board should I get?", "How do I stop?" or "What's wedging and how do I do it?"
It really comes down to personal preference. I like how centersets slide, and how long they last. They also seem to grip well enough for me, whereas BigZigs grip awesomely, but their slide isn't as smooth, so if s/he is looking for a wheel that's good for both, he should probably go with something that has a better slide and are usable for DH.
To the OP: What sort of downhill runs are you going to be skating on these?
If you're going to slide a lot you should go with round lipped wheels for a smoother/more predictable slide.
As for bearing seat positioning, it's definitely a matter of personal preference. Personally I like TRUE sideset wheels (Zigs are all OFFset) as they have a much more progressive transition from grip to slide to grip; they cone, but if you rotate your wheels often you can easily keep it to a minimum (and it doesn't affect the ride, sometimes I even think coned wheels feel better).
Also, if you like to run your trucks VERY loose you might get wheelbite with wheels bigger than 70mm 'cause the cutouts on the DropSpeed are not the largest ever seen (I get light wheelbite with slalom-style sideset 76mm wheels).
That said, you have plenty of options to choose from.
So far on my Drop Speed I've liked very much (for the same use you're indicating) 66mm Strikers, Flashbacks and older Gravity SuperG's (73mm). This weekend i'm gonna give my new Gravity Drifters their first spin.
Oh, and I second the question "What kind of DH runs will you skate on these?".