The lounges at Gatwick will probably have been refurbushed into the Galleries concept as at Heathrow T5 by next year - although of course there are no guarantees. Both airports already feature First check-in areas (as you're Gold).

Heathrow flights to Larnaca are using B767s this summer, and Gatwick flights to Paphos are using B737s. The 767 obviously has a larger cabin - and it's the middle seats that squish, leaving a 3-2-3 configuration compared to 2-3 on the 737, which some find inconvenient if you like window seats.

My personal preference would be from Heathrow (simply because I've never flown from Gatwick), but it's hard to justify the extra money.