There is some debate on the "robustness" of the 36 and 54t "upgrades" vs the 18t. The more shallow engagement angle of the higher tooth count ratchets might be less robust....I believe the new 350 hybrid hubs are going to come 24t when they come out.

My Hybrid 350 hub has the 18t and I find it perfectly adequate as a road biker. I did have some trouble with engagement within my first 100 miles but my dealer cleaned some grease off the teeth, put it back together (in about 30 seconds) and all has been well since....

I weight 280#, so durability / robustness is a key factor for me.....