Both units are good and have gotten better by some rules in 6th. Beasts need an IC to help with tankshocks, but can at least regroup easier. I'm assuming the Baron, but they can be run without.
Couple big bonuses:
Hit and Run is SHARED now. Baron gives it to beasts, Hellions can give it to other ICs.
Stun Claw is great for isolating ICs and challenging them (if you have an assassin type IC to jump in).
Beasts can assault through cover like nothing else with the Baron (cover from terrain plus Stealth, ignore terrain, assault grenades from PGL = fun times)
Beastmasters are characters, so you can use them strategically in challenges to save Baron or remove dangerous ICs from combat (go ahead, kill my 1-attack model with your CC beast...)
Hellion's shooting is nothing to scoff at including Overwatch
Both can glance-kill tanks like nobody's business
Hammer of Wrath, while weaker for us, is still a nice bonus when you're talking 20 models
Fleet still good for assault units
Really, you can't go wrong with either. Beasts are overall more durable, but Hellions are generally scoring (which makes them instantly your best troop choice).
"I'm good at killing fat boys. I like killing fat boys." -Arya Stark