Welcome to Hordes!
First, let me recommend some reading.
Battlecollege is a site chock full of tactics articles that will help you get a feel for your two choices.
Second, see if you can borrow a friend's copy of Primal. It's got all of the core rules you'll need to get started plus army selection rules for all 4 factions and Minions.
Now, to answer your question. Trollbloods are one of the more resilient factions in all of Warmachine and Hordes. All of their infantry has Tough (basically a 5+ save, but most units don't even get this), and all of their warbeasts have Regeneration in some form. They deal their best damage in melee combat, but they're generally slow. This leads to the "brick" formation. Slowly advance with your toughest guys up front and close together until you get into melee, then kill everything. Most of their infantry are on medium bases, and so they can get quite expensive $$-wise. They do have some small based infantry, but they require some tactical savvy to use well.
Legion of Everblight is one of the fastest armies in the game. Between naturally high
SPD stats, effects that move friendly models, and various extra movement triggers, Legion can cover a lot of ground. They use this movement to get first strikes in melee or move to a more advantageous position for their considerable shooting prowess. Generally, Legion is one of more fragile armies, but they do have some durable models and defensive abilities along with some attrition abilities that make their models stick around longer than they should.
Regardless of which faction you choose, the starter box is a good buy. You'll get plenty of use out of all of the models contained therein.
