As a female diver inching up to 200 dives with a partner who loves to dive, my suggestion is go for the best diving you can get to. If you want a nice resort, a good 6 star resort in Thailand is always going to be better than any dive resort. If you want to encourage your GF to get really into diving, take her to the best dive areas you can, and show her some variety. I'm a great believer in, particularly if someone is just starting something, giving the best you can afford - they are much more likely to get hooked.
Now, to throw another spanner in the works, I'm going to suggest Borneo as well, and if you are going in April, consider Pulau Layang Layang - it is hammerhead shark season (when we were there we would get between 2 and 7 hammerheads in one out of two morning dives, and if no hammerheads, then always manta rays - amazing - still get goosebumps), plus it had the healthiest coral I have ever seen - beautiful fans, gorgeous soft corals and hard corals - lots of schools of fish, great macro (frogfish and denise pygmy seahorses) as well as pelagics. The resort is fine, but who cares when the underwater world is that amazing!
Also, the trick with Sipidan/Mabul is to find one of the more budget operators who let you book the dives in advance. You will definitely get at least 1 in 2 days on Sipidan. Scuba Junkies is a good bet as at still has aircon cottages on Mabul. That being said, I actually enjoyed the diving at Mabul more than Sipidan and we gave away some of our Sipidan days - and I don't even carry a camera. Except for the school of chevron barracuda, Layang was more impressive than Sipidan on every count.
Whatever you chose, enjoy your trip!