I have a sail arm servo as I prefer them to drums. They are much faster than drums and there is no chance of tangles. It is all below deck, neat and tidy. But you do need a sail arm servo, as the standard servo will not have enough torque when the wind gets up. 7kg/cm torque should be enough 9 kg/cm is better. I know they are more expensive, but consider it as the cost of a few beers, and remember that once built your sailing is free.
A drum below deck requires a tensioner, made from elastic, however there is not a lot of room under the hatches to fiddle about with this, I hope you have small hands.
If you go above deck with a continous loop you will need to install pulleys, and this costs extra, it is visible as well.