You also might want to consider what Micronesia is like in July-August and whether you want to spend all that money to get there at what may be a suboptimal time for diving. Those months are smack in the middle of the rainy season and also at the beginning of typhoon season, though the weather there is generally unpredictable in any case. Topside, make sure you're staying in a hotel or on a boat with great a/c. I've been to Palau in March and May and both months were extremely hot and humid, May especially so when the tradewinds stopped blowing during the middle of the trip. I can't even imagine what it would be like in the summer months.
My trip to Yap was in the month of October and I recall it being somewhat comfortable on land, but that was a long time ago. The manta dives in Yap were almost disappointing - a couple mantas finally came through after a long wait at an otherwise boring spot of sand in the middle of one of the channels. You're better off with the Kona night dive IMO if mantas are your primary purpose, or better yet rescheduling your Yap trip to the manta mating season which runs Dec-Mar IIRC. I second Christi's point on expectations leading to disappointment. There are plenty of other reasons to enjoy Yap, however, besides the mantas.
On the other hand, I've had excellent diving in Cozumel in July. You can't go wrong for the price.