Online Pro: You don't have to keep track of a ticket to cash a winner; books do it for you
Landbase Pro: Assuming you are not betting with a local, you can feel slightly 'safer' in getting paid, knowing that you are standing in the casino, so it's not as though they're really a fly by night operation.
I imagine the answer is very different for sports wagering, horse racing, casino gambling and poker.
I prefer land-based poker, more fish.
Land-based horse racing is as much about the experience as betting -- but off-track land-based I don't really get. Online horse software sucks pretty bad.
Online casinos (and poker rooms) offer significant "deposit" bonuses -- don't find that land-based.