I'm just getting into the game myself, so I can't really tell you much about how each faction plays, etc but in terms of checking out casters, units, play styles etc this website has been invaluable to me:
http://battlecollege.wikispaces.com/
Also the
PP forums are helpful too. In terms of Warmachine, you generally aren't going to be running that many jacks so if there's a faction you really like except some of the jacks (Cryx, Retribution) then you wouldn't use many anyway. Play styles that I know about:
Circle (one of my chosen factions) are all about hitting fast and avoiding the hit back, terrain manipulation, teleportation beast shenanigans, all fun stuff.
Cygnar (another of my factions) are mixed arms, maybe tipping towards shooty, each unit is specialised, and they use a lot of Mercs (so there's your Mercenaries :p).
Legion you can play very beast heavy (they're the best faction for beast heavy lists, mainly due to the shepherds) so the fact their infantry looks like ass isn't necessarily an issue. They're the Alpha Strike/ Glass Cannon faction of Hordes, like I think Cryx are in
WM, they hit hard, but will fold when hit back. A few of their beasts are ranged too.
Protectorate are all about fire so I'd skip them if you don't like fire
Mercs have 5 different 'Contracts' you can play; 4 Star Syndicate is like the bad guy Mercs, Highborne Covenant is the good guy Mercs, Puppet Masters is the Cephalyx, they're really small
atm, kinda like Haemonculi from
40k Dark Eldar, experimenting and altering humans. They don't use Jacks but instead Monstrosities, which don't have a cortex, have low armour but a full damage grid. The Seaforge Comission is the Dwarves of the setting, I know their jacks are very robust and their colossal is very very good, and Talion Charter is the Pirates. I think all the merc forces (bar Cephalyx) are pretty mixed arms, just different flavours.
Convergence, I'm pretty sure all the models they have now are all the models they're gonna get, so they might lack a bit of versatility.
Retribution are combined arms also I think. I don't know much about them other than they're racist elves and they look cool.
Hope this helps in some way, I might be wrong on a couple of bits as I'm just starting myself, so someone else can correct anything that's wrong
I haven't voted on the poll because I don't know which you should go with haha, had enough trouble picking my factions, and I'm doing 4...