Heya. I play both. I play a lot on Battle.net, mainly because I have a bad habit of starting many characters. :)
Haven't heard of the synergies or the unlimited stash, are these hacks?
I'm up for a game anytime.
The synergies are where points in one skill strengthen another skill, for example as a Paladin putting points in sacrifice will add 12% per point to zeal damage, there are many other synergies. Mainly it helps to add more variety to builds as not make them as "cookie cutter" and makes use of those wasted unused skills. It's a new feature that came with the 1.10 patch, you have them if you are playing on Battlenet.
The unlimited stash is one of the features of ATMA, a third party muling program. It has a number of other features like a bank that you can share amoung characters and a drop calculator. It is for SP only.
http://atma.diabloii.net/