Sell me on Hellions or beastmasters

Remember the Baron is good to go with the Beastmasters now that they move at the same distance in the movement phase, all have fleet for the assault, and you only have to ensure that the Baron him self doesn't assault through terrain so he doesn't force the whole unit to take a differcult terrain test on the charge.

If you're up for converting the beastmaster unit I'd say go for them, if not go for hellions.

Rathstar

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).

hellions, when using the right drugs and can get a charge off, can hit like a truck.

but beastmasters with khymera can chew the face off a daemon prince.

Beastmasters have probably benefitted most of our assault units from the new rules, especially with Baron. Thye move 12'' now and aren't slowed down by terrain, Beastmasters are characters so you can sacrifice them to keep enemy chracters from hitting anything valuable. Baron gives them hit and run now and running him in front of the unit allows for some nice wound allocation shenanigance: he takes every wound amduses look out sire (on 2+ since he's an IC) to pass them on. Since the wounds redistributed by LOS are allocated by the owning player, you can place s5 or lower on Razorwings and s6 or higher on Khymerae (or a Fiend if you're running one). If you fail the LOS roll, you still have a 2++ save so the chanses of Baron getting hurt are very low.