Re: What exactly is a "pure Android experience"
Most phone companies running android put their own skins/modifications to android. Samsung is infamous for this, and they call it Touchwiz. Due to the heavy modifications, updates take longer because Samsung needs to modify the code to fit Touchwiz (same goes for any phone with UI skins). Additionally, added skins slow down the performance, but not severely. With a phone like the Nexus, Moto x, and the droid (not pure android but the mods are very minimalistic) the Android is just that, android with not addition it it. In turn the updates arrive faster (same day for Nexus phones) and the phone itself runs smoother. Also, developing roots is easier on pure android. There is definitely more to it but that's about all I can think of right at the moment. I'm sure someone will chime in with more info.
I'm definitely getting the nexus for the stock android. Or maybe the Moto X, we'll see when the time comes.