I'd steer clear of the 500r I have one that I wasted my cash on it allows quite massive variations. The others under the 100$ make have a 2 degree +/- probe error just plan that in when you set it as it could be 2 degrees hi or low of what you use. 90 is nice and safe (88-92 true) Rheostats work great as long as you don't have large room temp swings 5º is as big as I like to see. Personally for the extra 60 bucks I'd go with a proportional T-stat and sleep well but that is me.