my best advice - stay away from digital it always fails at the worst possible moment. to much accuracy will also cause weak chicks. I myself am partial to wafer thermostats they are pretty accurate and hardly ever break down. I do keep an extra one on hand just in case, but its more insurance since I hatch other people's eggs too.
you don't need the best in temperature controls, a degree of variance is usually fine.
keep in mind a hen does not have a thermometer, a hygrometer, or a wristwatch. ive never seen one write on a calendar either.
another thing to think about- ancient Egyptians hatched eggs in cave like structures, heated by open fire. they more than likely controlled the temperature by "feel".