I haven't tried using Maptools, or GoogleDocs draw function. For maps I just use a spreadsheet with colors and letters for PC's/NPC's then take a screenshot--it's easy to manipulate, big as you need it to be, and it already has gridlines. May have to try one of the other ideas though just for a change.
Group initiative is a must if you don't mind my saying so. I've tried one or two combats on here using individual initiative and it took the better part of a month for a 1st level combat that only had 3 good guys and 3 bad guys. And people were posting almost every day too. I would also recommend you and your players put as many rolls into a single post as you can. E.g., roll To Hit and Damage at the same time, then adjudicate later. It does save time. Given how that works, I highly
recommend story-driven experience as several others have already. But I'm biased towards a plot-heavy game instead of combat heavy.
Also, dice tend to get in the way of a lot of plot and RP, IMO. Don't forget that Passive checks are there, and perhaps consider using Passive Checks for Trained Skills to introduce plot details without having to wait on the entire party to roll a check on. It definitely speeds things up.
Be aware of how long it takes people to post, react to one another, and for you to get back to them. Large parties (6+) are difficult to deal with, as I've found out--running Eberron with 7 right now. Combat can be a nightmare, as are some conversations just due to RL often keeping people from posting more than twice a week, and others waiting to get in on the action. 5 in a party is a good round number.