My understanding is if that if you get on guestlist you don't have to pay cover but have to be at the club early? Whereas if you buy tickets you can show up whenever? My girlfriend and will probably buy tickets but does this guarantee entrance?
Yes pre sale tickets are guaranteed entry and have their own line which is way smaller than guest list.
Tickets will typically guarantee entry until midnight. At least that's the case with Hakkasan and Omnia.