I'm not sure if there is a real difference between the script managers.
I simply use the telerik but if the MS thing has positive aspects why not use this.
--Maybe someone has better knowledge about the differences.
Asmx is (very) simple and straight.
WCF has much more options ("power").
So from my point of view:
If your services will only be used from your pages take what you prefer.
But if you plan (or could think about a possibility) to add other "service client" later I would prefer to use WCF.
A last few words about the "postback without state information".
Maybe Dino Esposito's article can point you in a different direction:
And you maybe also think about user / custom controls. The idea is to encapsulate functionality into a control which does it's job without any special needs for the containing page. I think about something like the telerik combobox which can pull data from the server when it is needed without transferring the viewstate.