The difference between varks renegades and
IA13 renegades in short:
Varks are only nurgle and khorne,
IA13 have all 4 gods.
Varks have some neat
FOC options, while
IA13 has the relic tanks.
Of the possible devotions:
Rouge Witch-psyker
HQ can can spam psyker squads, a bit cheaper at varks, unfortunatly renegade psykers suck.
Mutant overlord-unique to
IA13, the "melee assault horde" devotion, plays a bit like orks.
Master of the Horde-resurrecting lesser infantry spam, varks do it better thanks to unique
FOC (unless you wanted T or S dudes)
Arch-Heretic-zealot dudes, lots of enforcers ("commisars"), dual god choice.
Heretek Magus-
IA13 unique "darkmech light", makes your army a bit like chaos admech with walker acess, and
FnP cybernetics.
Bloody Reaver-
IA13 unique, much more like proffesional renegade
IG than others, also has scions equivalents.
Ordnance Tyrant-varks unique, all the artillery you'll ever want. (far more than even
DkoK can muster)
Shock Legion Taskmaster-varks unique, ogryn assault master. (a bit odd one)
Before you dive into shopping spree, do yourself a favor and read this:
http://1d4chan.org/wiki/Warhammer_40,000/Tactics/Renegades_and_Heretics%287E%29
It will make it easier to tell what you want, as there are TONS of options. renegades is a very versatile army that can go many, MANY viable paths, far more than their loyalist counterparts.
EDIT:
BTW, in an odd twist of fate, the sentinal who is so god aweful for
IG? probably the strongest heretic unit. a sentinal with a heavy flamer is mere 20 points, the
BS reduced to 2 is irrelevant, it has a template gun anyway. (and even its lousy 10/10/10/2hp profile is nuts at mere 20 points.). no matter where you go, I'd suggest taking a good look on the possibility of running large numbers of these buggers.