Tractors are easier on your butt and back because they don't bump you to death. Beware of used riders on Craigslist. Make sure the seller can easily start it cold and lets you mow a few strips with it. Look to see if it's cutting even and listen for bad rattles, knocks and squeaks. Pull the dipstick and look at the oil. It could tell a story of non-care that might make you pass up the deal.
If you can buy one from your cousin or neighbor who has never given you reason to mistrust him or from a close personal friend, you may do great. If you buy one from Fred down the street who "works on stuff" you may not be happy with the end result. I sell one rider out of 15 that I take in. I scrap most because when someone brings one back to me, I work on it for free because of this stupid conscience thingy. Not worth the hassle, so I strip them, sell some things (starter, carb) on EBay and give my scrap guy the rest. Sometimes I save the wheels and tires for neighborhood kids who will take them home to make the go kart that never gets made. I'm sure their parents love me.