Hello Hero. Welcome to the forum, good to have you aboard!
Two things to consider. Weather/sea conditions, and what your fiancee is interested in.
Phi Phi offers the better snorkeling, and you, along with a lot of other people get to see Maya Bay and also some beautiful white sand beaches. Phi Phi trips however are the first to get cancelled in rough weather, or worse they are often not cancelled when they should be. In July there is often rough weather.
Trips to Phang Nga Bay can have a bit of snorkeling, although most don't, and can also include canoeing, entering the hidden lagoons (hong), a look at James Bond Island (I wouldn't bother going ashore, too crowded, but each to their own), a visit the Kho Panyi the village on stilts and the most incredible scenery. The snorkeling is not as good as Phi Phi, however it is a good spot for beginners.
Have a look at 'things to do' up the top there, and then go to 'activities' to see reviews of the places and the companies that can take you there.
Happy holiday planning.
Phuket Sail Tours.
Edited: 10:48 pm, June 17, 2013