With regard to jetbikes, I think Reavers are much better for several reasons. Perhaps most importantly, cost. I can have 6 Reavers for less than it costs me to buy 2 Skyreavers. And, I'm a big fan of having more bodies on the field.
Also, Reavers seem far better suited to the current jetbike rules - in that they're primarily a combat unit and so can freely jink for a 3+ cover save (plus they can get FNP). Skyreavers meanwhile... ugh. Why are virtually all their ranged weapons blasts? It means that if they jink, they can't even snapshot. Instead, they're relying on a 5++ - which really isn't something I want on a T4, 50pt model. I think they can get a 4++ for one turn, but even then it seems laughably easy to just torrent them down.
With regard to Incubi vs Harlequins, I'm unsure. Honestly, I don't really like either. Both of them seem too fragile for their cost, and the Incubi's lack of grenades kills them completely for me. If I had to choose one, I'd probably lean towards the Harlequins. But, I really can't see myself taking either.