Just looking at that map you can tell who the better fighters are, Kurds have pretty neat and organised battle lines while the rest is just a giant clusterfuck, ISIS just seem to be using pure aggression, not surprising since it's reported on they use drugs a lot in battle and so on.
It has little to do with "quality" of the fighters
Progovernment Syrians want back their territorial integrity, so defending even isolated locations among sea of barbarians make political (if not tactical) sense for them.
Democratic opposition is laughing matter since 2012, they are openly collaborating with ISIS.
Kurds are mostly (and cunnigly) operating in their own ethnic areas, therefore avoiding costly guerilla warfare.
ISIS is by now the most powerful force in both Syria and Iraq, it is not locked in by any superior military, but by extent of sunni Arab settlement - its spine.
I am sceptical on ability of Assads troops to keep the Deir er-Zor, its classic siege operation and they are not getting support from outside. They are dead meat. Taking the base is obvious choice for ISIS as islamists need "breathing room" without having to worry about (even if isolated) trouble makers.