Ok. Here goes.
SM- A small model count, more $$ for you. Lots of really tough dudes in easy-to-paint power armor. Limited color palette means ease of painting.
Tau-A medium model count, some $$ for you. Lots of moderately tough troops, very good at range. Battlesuits, Hammerheads, Seeker Missiles. Easy to paint unless you go Kroot.
Eldar- A medium model count, some $$ for you. Lots of cheesy (READ: CHEESY) toys to protect your not-so-tough-without-psychic-powers pointy eads'. Easy to paint due to usual two-color-scheme.
DE- A medium-to-low model count, however, ungodly expensive. REALLY kick-ass CC units backed up by Fast skimmers and cheese toys. Lots of fun when you realize you have twice as many nasty CC monsters as he does.
Tyranids: High HIGH model count, low $$ for you. A massive gribbly horde able to smush pretty much anything. Infantry able to take on tanks...Huzzah! Backed up by equally gribbly monsters that are hella fun to build. Just make sure you have paint for teeth.
Necrons: Low model count, more $$ for you. Gauss weapons able to destroy anything, reallly. Massively powerful weapons mounted on light skimmers, and Lords with lots of kool toys (Read Resurrection Orb). And the Monolith, the second best tank in the game.
IG: High model count, not much $$ for you. Tons of Heavy Gunz availible. Tanks Tanks Tanks! Also, watch out for the Plastic Valkyrie (which of course is unconfirmed but expected none the less.)
I gotta go. I'll finish this later.
-Dirge