i sugest you go dive both locations because you obviously have no clue what you are talking about or you are just trying to promote the bali shop you obviously work at.
I HAVE dived both areas, as a friend has a dive shop on Gili T and we had planned to do 4 days intensive diving there. However, after just one dive, where we saw just about nothing to write home about, even compared to our local dive site of Blue Lagoon in Padang Bai, we sacked the diving and just chilled instead.
There are many people who will say that Gili T diving is great, and maybe there are good dive sites, but from my experience, I don't see how it even compares to Bali diving. I would say that Gili's is great for learning to dive, but for experienced divers, I think they would be disappointed. Just my opinion.
