Offline - Easier to find businesses who have been around for many years, have healthy profit, and are already spending in the xx,xxx a month on advertising. They are also less likelier to be saavy about anything online let alone SEO. You can price your services for a lot more, build a real and ongoing business relationship, and generally make more money.
Online - You are running the gammut of people who want to be #1 on Google, Google Place, and get rid of all negative feedback for something like $75 bucks a month. They are going to annoy the piss out of you by emailing and calling you everyday. Oh yeah they are gonna wanna pay by Paypal and chargeback your ass the instant they feel neglected