If you buy weaned lambs (more expensive but will fatten on grass) you will only need to check them once a day but pigs will need feeding at least twice a day. They will need water containers constantly topped up where lambs drink very little. Lambs will not burrow a way out as long as you have decent fences/ hedges and will not root up your land to the point where you have to get it ploughed and re-seeded!!
Having said that, I like pork more than lamb and you get more meat per animal!