series1, series2 and series2.5 radio modem, which is better?
?
Better in what way?
planing to buy pair of moudules for learning purposes, which will be better?
Decide yourself, some of then have more or less range, and the pro ones support mesh networks.
series1, series2 and series2.5 radio modem, which is better?
A hammer, an oil lamp, and a paint brush. Which one is better?
None, of course. Each is completely useless. You can't paint with a hammer. You can't drive nails with an oil lamp. You can't read by the light of a paint brush,
The Series 1 and Series 2/2.5 modules are for different purposes. Are you trying to paint, read, or pound nails?
A couple links below that may help. You'll probably find a lot of stuff on the web for S1. But there is also some for S2, and Robert Faludi's book uses S2. If you're looking for learning resources, I recommend that one.
http://www.digi.com/support/kbase/kbaseresultdetl.jsp?id=2213