In a competitive environment, Froslass is better due to its versatility; however, Glalie has certain strengths too.
PROS OF FROSLASS:
- Secondary Ghost typing gives it more offensive coverage, with both Ice and Ghost being superb offensive types.
- Base 110 Speed outspeeds major threats, including Garchomp, Landorus-T, and Tapu Lele that Froslass can threaten (to some extent)
- Has access to Destiny Bond, allowing it to be an effective suicide lead with Spikes; it also has Will-O-Wisp and Thunder Wave along with Hex
- Can feasibly run a physical set with Triple Axel and Poltergeist or a special set with Ice Beam and Shadow Ball, along with Trick to cripple a defensive Pokemon with a Choice item (Band on physical sets and Specs on special sets)
CONS OF FROSLASS:
- Slightly frailer, going from 80/80/80 bulk on Glalie to 70/70/70 bulk, with the 30 stat points going into the aforementioned Speed.
- Abilities are "useful", but Snow Cloak is banned in most competitive formats and Cursed Body is situational
PROS OF GLALIE:
- Slightly bulkier, as mentioned previously
- Has access to Moody, which is broken if allowed
- Has less weaknesses thanks to not having a secondary type
CONS OF GLALIE:
- Slow, and 80/80/80 bulk does not justify it being just 80 Speed (very bad)
- Does many things worse than any other Ice type
- No secondary offensive STAB, so it struggles to hit anything not weak to Ice
- If a physical set is run, it doesn't have Triple Axel and is forced to run Ice Spinner, which is 33% weaker.
- Both abilities that are not Moody are bad
Overall, Froslass's pros outweight its cons. Being fast gives it an incredible advantage over Glalie, and the expanded support movepool helps it as well. Although neither can hit incredibly hard, Froslass can at least set up one or two layers of Spikes with its Speed and cripple something with status or faint an opponent Pokemon with Destiny Bond. Glalie, being slow, does not function as well in the lead slot, and it doesn't have the utility options to make it up. Even in terms of offensive prowess, Froslass has an extra offensive STAB as well as stronger moves on the physical side, allowing it to run Choice Band with ease. Glalie does have Moody, but if you're playing competitive, it is banned, meaning Froslass's legal ability in Cursed Body is generally more useful than Glalie's best legal ability Ice Body. Although Froslass does have more weaknesses, having 5 compared to Glalie's 4, the offensive nature (and natural frailty) of Froslass means the extra weaknesses doesn't affect it too much, as it is trying to be either a glass cannon or a suicide lead. Glalie, although having that one less weakness, fails to compensate for the its multitude of flaws.
To be fair, there are better Ice types to be used, like Weavile, but between Froslass and Glalie, Froslass is superior.
Hope I helped!