ZIPAIR Full Flat is probably the best "premium LCC" experience out there, with free Wi-Fi, entertainment and in-seat power offsetting the usual nickel-and-diming annoyances
If the location and lack of facilities aren't a deal-breaker, then the spacious rooms, modern design and excellent breakfast are great reasons to stay at Mesm.
Passengers departing from Changi Airport are free to clear immigration from whichever terminal they please (with one small exception that won't matter to most).