Purchase + Subscription. I realize it's not everyone's favorite model, but the devs make it worth it. They plan (and so far have successfully implemented) to release an "Ultradrop" at the end of each month filled with loads of extra content. The end of the first month (last month) they released a drop with two new level 50 zones, a bunch of costume and decor stuff, and a plethora of general fixes. The drop at the end of this month has already been revealed to include a new PvP Battleground and a bunch of yet-to-be-announced content as well.
Plus if you find you're making a ton of money in the game, you can buy game time with in-game currency from other players thanks to the C.R.E.D.D system (This also works in the opposite direction, i.e. if you have extra cash you can buy game time and sell it to players in-game)
I actually prefer a subscription model to one based around microtransactions (not to mention double dipping with microtransactions AND a subscription which is a major no-no in my book) because at the very least it puts everyone on even ground without making you feel like you're missing out on something unless you pay up. Obviously, its not my favourite model by a long shot, but I can live with it as long as the monthly fee isn't ridiculous.
How are they handling payments during the beta phase. Subscription model already in place or is it "free" until the game goes gold?