I thru-hiked the AT to Maine, then a couple of years later thru-biked the Northern Tier route to Maine. I spent plenty of time pondering the difference between the two trips.
Free camping was one big difference. You can tell by the posts above that camping by bike in much of the urban, agricultural and pastoral Northeast can be problematic. It's much easier on the AT. Walking is also much simpler, and you don't have to carry stuff to support the bike. Walking is cleaner, too.
But if you agree that riding a bike is just plain fun, well then, all the little problems are worth it! And if you like a restaurant meal every day or two, biking is for you. And of course you don't have to carry much more than a picnic lunch and some snacks, and very little water.
By the way, warmshowers.org was one of the joys of my trip through there. I only used it a handful of times, but I met some great people. It seems fairly well self-policed. You can read comments by and about hosts and guests. They seemed like honest reviews.
I only found a few good stealth sites from NY to ME, and one of them was very close to an AT trailhead. Another was a town park, with permission. Pay sites were very expensive, about $35 for a tent site. I found a couple of $50+ motels in the same areas, so for the price difference I stayed in more motels in that area than usual.