I'm in the campden/sorbate camp. After fermentation is finished, rack the cider into a carboy with 1 dissolved campden tablet per gallon, and 2/3 tsp sorbate per gallon. (But, if you're doing 5 gallons, don't go over 1.5 tsp total on the sorbate). Let that sit about three days, then sweeten to taste and rack to the keg.
When I sweeten, I like to sweeten the sample I have to taste. Then, take the SG of that sample, and then sweeten the whole batch to that SG reading. I have some free software (winecalc) that tells you if the FG of the batch is 1.000, it'll take 18 ounces of sugar to bring it to 1.010, for example. That way it's easier to sweeten the batch to a good, not too sweet, level.