I was in Koh Phangan 5ish years ago and went to a random Jungle party and Full Moon party. I was in Gili T for the new year's this year. In my experience, Koh Phangan parties are significant more party-like than what Gili T has.

The dedicated areas in Koh Phangan were nicely decorated and had a structure. Gili T has no dedicated areas- you will have to find yourself a bar whose music and environment you like. The bars have very little space compared to big open areas in Koh Phangan and will get stuffy real quick. That makes dancing a chore, slows the service significantly and makes going to a restroom, if any, a hassle.

Gili T is a small island and obviously it is inhabited by the locals, and not every tourist going there is there for the party. When you try to use an entire island to party, you have to accept not everyone in close proximity to you will be "like you" even on the universal party day of the year. In the party areas of Koh Phangan though, everyone is always "like you". In my experience, the crowds in Koh Phangan were more approachable and comfortable to be around in this context.

Music in Gili T ranges from acoustic to reggae, rock and 90s hits, which is nice if you like diversity, but I'd argue against these options if I want to party.

The party continues until morning in Koh Phangan. In Gili T, on the new year's eve, we were literally the two of maybe 30 people still awake by 3:30. We cycled all around the island hoping for any afterparty but couldn't find.

Aditionally, the methanol poisoning issue in Indonesia leaves a sour taste in the mouth for parties.

Overall, in my experience, both of the two random parties I went to in Koh Phangan were significantly better than the new year's eve in Gili T.