For short, two-three-episode stints? Both. Misty was always one of my favorite characters from the original series, and it's always quite nice to see her when her personality isn't being brutally murdered by having nothing to do. It'd be cool to see May, too, if only to see how much she's progressed as a coordinator since she and Ash parted ways. I actually think that she could make a neat "unbeatable rival" a la Robert and Soledad in the post-Sinnoh arc if there is one, but it would be nice to see her before then.
But for a place in the main cast? Neither. I'd much rather not see Misty's personality go through what it went through in Johto again. The writers would have to come up with something exclusively for her, something that only she could possibly do--but when the only thing that really sets her goal apart from Ash's is that hers is type-specific, and when there's already a more experienced battle-oriented character on the cast, what could she possibly do that wouldn't be a new Gym quest, except with only Water-types? Not to mention that it was exceedingly rare for her battle strategies to consist of something other than "Water Gun/Spike Cannon over and over again!"; can you say "boring"?
It also wouldn't work to bring her back for her personality. While I'm sure that hotheaded tomboy is still in there somewhere, there's no ignoring the fact that her time as a surrogate mother to Togepi and her time as a full-time Gym Leader have helped to mellow her out considerably. Someone mentioned earlier in the thread that there wouldn't be half as much arguing between her and Ash, if that's why some people want her back. However, even ignoring Misty's development, you have to remember that Ash has matured as well; he's no longer adverse to accepting friendly advice, and as we've seen with Paul, he'll ignore snide remarks when it really counts. The writers haven't given us much of any Three Stooges-esque slapstick in years, either, so that part of her original personality is out as well. (I miss you, random mallet... D: )
And besides, she appears at the least to be far more competent as a Gym Leader than her sisters; remember how things nearly fell apart when she visited Pallet Town at the start of the Battle Frontier? It seems it's pretty much necessary for her to keep a constant watch on the place to make sure her sisters don't start giving away badges for free. Put simply, it's a responsibility that she just can't shirk, especially for upwards of a year.
As for May, well, there's the obvious matter of the writers not wanting two people with the same goal on the main cast. Other than that, though...honestly, I think that the episodes leading up to and including her departure were the peak of her development as a character. Between the end of Hoenn and the Kanto Grand Festival, she received next to no character development. If things hadn't changed, if May hadn't realized that she'd been influenced by Ash's battling style since day one and that she was being held back as a result, things would have simply continued on like before. May's character would have continued to stagnate like Misty's did. At this point, for her to suddenly up and return would, as someone else said earlier, be contradictory to what she said when she left. Besides, I highly doubt that she'd start a new quest in the event that she's already registered elsewhere.