Which is better and why?
Depends on what you are using them for. Can you clarify the question? Otherwise, I can list the pros and cons of each other in a comparison and let you decide for yourself. But again, best is subjective, it depends on what you are looking to get out of it.
D. Glalie and D. Froslass have similar stats, with D. Glalie being well rounded but slightly under average in all it’s stats, making it more or less useless. The only advantage it has is it’s versatility. D. Froslass on the other hand, is a decent mixed attacker with slightly lower defenses than D. Glalie, but with those stat points transferring to speed, which is why it finds some small use as a support, setting SR and Will-o-Wisping opponents. It also gets Taunt, Haze, Spikes, Knock Off and Pain Split, allowing it to become even more of a nuisance. It could potentially be used as a Baton Passer, with Flame Charge to boost Speed while being affected by Taunt, until fast enough to Baton Pass when it wears off. It doesn’t have any other notable stat boosting moves to help with this, so it would still have to run Haze/Taunt and the like.
Glalie’s typing leaves it with much to be desired, with 4 weaknesses, notably the ground and water weaknesses. Froslass’s mono fire typing is not bad, but nothing special about it either. M.D. Froslass has it’s typing shared by Chandelure, but is overall better at it’s job, having higher speed and SpA at the cost of an item slot. It is a much better typing offensively and defensively than Glalie’s so score for that too.
Will compare the Mega formes later.
I’d go with δFroslass personally but I follow more the aesthethic rather than the performance and δGlalie looks (to me) more like a bug with a lot of honey rather than magma.
Statwise, Mega Glalie gets a huge boost to it’s attack and special attack, and a small boost to it’s speed. It’s defenses remain subpar tho. It works well as a mixed special attacker, but is more powerful on the physical side due to Reckless, which boosts it’s Flare Blitz and Head Smash, but leaves it is a double edged sword, as it takes more recoil due to losing rock head. If mega evolving, the recommended ability is Flash Fire, to swap into a predicted fire type attack, then spam flare blitz until dead or needing to switch out for wish support.
Mega D. Froslass, on the other hand, becomes moderately bulky on the special side, and has a massive boost to it’s special attack and speed, making it a viable sweeper. It’s recommended non mega ability is either flash fire or competitive, to swap into a defog or intimidate and boost or a predicted fire type move. It gains the ability magic guard, making it happy to take a toxic and sponge status. it also removes it’s sr weakness. It also has a decent movepool of special moves and has a good offensive typing. it has shadow ball, flamethrower, energy ball, dazzling gleam, and eruption to name a few. overall competitively, froslass is much better, now that glalie has lost moody.
My pleasure, always fun analyzing mons.