Accommodation-wise, those are two totally different places; Cha Cha is very nice (for Bunaken standards), in that it has hot water, A/C, but no pool or other major amenities.
Gangga resort is "top class" in that is has a pool, gorgeous beach, a la carte menus, massage/spa, gaming room, etc..but it is also more than double the price...so indeed you "get what you pay for".
Not knowing you personally, but based upon your preference for Kasawari in Lembeh, I'd think Gangga is more to your liking, as far as accommodation. But again, I am just basing this on what info. you have given in your post (you didn't mention a budget, or what amenities you want/need, etc..)
Diving wise : some similarities between the two spots in that both have "pretty" corals, excellent vis, possibly shark, napoleon wrasse, bumphead sightings, etc...however Bunaken is primarily wall diving, while Gangga has more gentle sloping reefs, white sandy bottoms, and probably cleaner/more pristine conditions overall.
Both are subject to currents in July (depending on tides, dive site, time of day, etc..) and both offer decent snorkeling--however, surface trash is commonly seen around Bunaken while at Gangga, much less-- if any at all.
Certainly the "remote-ness" gives Gangga an added air of "exclusivity" which is desirable for some--and were it my honeymoon, or other "once in a lifetime" occasion, I'd splurge for it myself. But ultimately, it depends on how much of your total satisfaction is about the diving (alone) and how much is about the rest of your overall holiday experience.
They are both nice, but different. Sorry if I just added further to your confusion!
HB