From my experience...neither. Horde has more people (barely) so you'd think they would have more pve. However, this isn't the case, unless you want max GS ICC or RS non stop.
What about Ally? Well, as an astute player said above, the mentality here is "PvP is ONLY for Ally (TM)". Now, do they do PVE? Yes. As much as Horde, actually, which is really just code for...you guessed it...max GS ICC and RS.
Now, if you stay low level (under 60), sure, both sides will pop frequently. After that...well, better learn to solo content.