Hi Denise
July/August is the good season weather-wise, which of course means it's also the busiest time of the year...
By staying in Labuan Bajo you will get much cheaper diving than going on the liveabaords, but you wont dive as much. Most day trip operators do 2 dives a day, sometimes 3. On a liveaboard you usually get 4 dives a day. And you get to do night dives. One night dive in particular is amazing, Wainilu. It's great for macro, decorator crabs everywhere!
I guess it depends on how much time you have. I suppose you'll see more dive sites on a liveaboard, because some sites are quite far to do on a day trip (2 hours or more to reach the site). Many sites need to be dived a specific time as well (rising tide, falling tide or slack). Being on a liveabaord you'll get more time to plan your dives, as opposed to day trips where you have to follow a fixed schedule.
Staying on Kanawa can be nice, nicer than in Labuan Bajo but also more expensive.
I wont name specific operators as I work here and I dont want to be seen as advertising for the place I work at.
The prices for day diving tend to be the same everywhere. Liveaboard prices vary, you can get great discounts without booking ahead and just asking directly at the offices in Labuan Bajo, but you get the risk of all boats being full or, depending when you come, that none of the boats have enough guests to leave.
Read what you can online to decide which boat to go for. There are trip reviews here on Scubaboard, tripadvisor or other dive forums.
Hope you'll have a great time here, you've been here before so you know what to expect

---------- Post added February 14th, 2013 at 07:29 PM ----------
Dugong
June is also a good time, the visibility is nice and it's not as busy as July/August. Still, it's not low season so dont expect to be alone on the boat.
As I said before I dont want to mention operators. I'm only really familiar with the one I work for anyway (in terms of gear, cabin design and all that). Most of the dive centers have local dive guides who know the sites very well. I'd say the main differences between the operators will be in terms of facilities on the boat and what is and what isnt included in the price (soft drinks, equipment rental, nitrox ect). Gear quality might change as well, in case you dont have your own gear that might be important. Read some reviews or PM me if you want more information.
Enjoy your time there, it's great!
Oh one last thing, I see you dont have a lot of experience in diving yet. Many sites have very strong current here. If you don't like current, you might want to look at another destination. There is often 2 ways to dive a site, one with less current than the other, but at times you cant avoid it. And at times you want to be IN the current, like in Castle Rock. Much more life!