Personally, I would say that is sometimes an advantage.  People can get pretty worked up sometimes about a hot week here or there when sometimes it really is just a hot streak or a cold streak and really not very meaningful.  I mean, remember all of the hand-wringing about how Cuddyer was never going to pan out, since he always struggled in April?  And about how David Ortiz probably wouldn't be all that good because he was so inconsistent from month-to-month?

Roberts has been striking out a little less with the Twins than with the Reds, but it's been a very small decrease, and is still striking out twice as often as he walks, even on this hot streak.  At any rate, there's nothing in his Twins stat line that is different enough from his Reds stat line to convince me he's turned any sort of corner.  Maybe he has, but it hasn't shown up in his underlying performance indicators so far.

Now, if you can give me reliable scouting info that says Roberts is a much better defender than Span, then I'm willing to listen.  I've never been sold on Span as a good CF since the only thing you ever hear about him is he's fast, which is far from the only thing required to play CF well (as you've mentioned before).

But right now, it doesn't look to me like Roberts is clearly better.  Maybe somewhat better, but clearly better is a stretch.