Gear drives are a little more accurate in terms of cam timing than a chain drive, but the difference is much less with the latest system from Harley versus the sloppy system from '99-'06. Unless you plan to put the bike on the race track and thus need every tenth of a horsepower, it's not a valid consideration. But the argument does sell lots of parts.
The biggest factor on late model ('07 and later) bikes is the sloppy crank. And it's not good enough to just measure the current runout and if it's .002" or better (fat chance) think you can just throw in the gears and forget about it. These cranks are subject to shifting at any time, so unless you plan to pull the crank and have it trued, plugged, welded, etc., I wouldn't even consider gear drives. A good up-to-date Harley engine guy should know this and not try to sell gear drives without explaining all of this. I can't tell you the motives of your guy, but I've run into more than a couple who still push gear drives just because that's what the hot setup was ten years ago so they assume it still is. Plus they make a lot of money on them, of course. After all, they are in business to make money, right?
Jerry