I currently play skaven (albeit not very well it seems

) and am currently expanding into Beastmen. They are similar in a number of ways, but really distinct from each other in play style.
Skaven
Pro:
- Versatile; can be shooty or choppy depending on choices
- Resilient; cheap troops can remain steadfast for several rounds when properly supported
- Strong magic; Cracks Call is the only thing I know of that can destroy buildings. Also: Dreaded 13th. Nuff said.
- Cheap Heroes; Warlocks weigh in at a paltry 15 points base. Granted, they're barely better than a clan rat...
- Warp Lightning Cannons

Contender for top war machine in the game.
- Weapon Teams blow up in hilarious ways
Con:
-
Ld issues if NOT properly supported
- High model count = $$ (IoB lessens this somewhat)
- Weak (but resilient) core. You need lots, but you rarely win here
- Some units are high cost / low return (Jezzails, I'm looking at you; Ranged units in the Special slot should at least be BS4...)
- Multiple HPAs may earn you a punch in the face...
- Weapon Teams blow up in hilarious ways
Beastmen
Pro:
- Strong
CC with Primal Fury
- Ungor blocks are decent anvil units to hold up attackers. Cheap, and come with
HW/S
- Decent special choices. Minos cause impact hits, get stronger with each winning round.
- Good magic item support. Shaman swarm w/ Herdstone generates good power dice
Con:
- No armor on core units means lots of dead models; rarely if ever get to strike first.
- Ambush special rule spotty at best
- Hard to use Rare slots. Won't say worthless, but close.
- Little to no shooting / range support
Overall I'd say Skaven are the stronger army. Their book is more recent than Beastmen, which was generally considered an underwhelming army book. Beastmen core choices tend to be more expensive than skaven, while not being a whole lot tougher (although a decent bit more killy). Skaven are a well rounded army as well, they have decent
CC in units like Rat Ogres or Plague Monks, good to godly shooting in Weapon Teams and Warp Lightning Cannons, tough monsters like the HPA (which would probably solo any Beastmen monster, none of which have a save or regen :( ) and while Beastmen can use a few of the better Main Book lores of magic, the lores of Plague and Ruin are amazing.
To be fair though, I haven't actually played a game with Beastmen yet.