All those brands are very good.
Ranco/Johnson are both on/off - meaning you set a temp and a difference. So the unit heat up to the temp then shuts off till the difference temp is met then turns back on (ie. you set 98 with a 3 degree difference. So it is on till it hits 98, shuts off till it cools down to 95 then turns back on)
Helix/Herpstat are proportional - they are basically always on but as it gets closer to the set temp is lowers the amount of power used...so in this way it holds the temps to a degree in most cases and uses less power over all.
All will work for your needs, Helix and Herpstat will be more accurate but will also be $50 - $70 more.
Hope this helps,
Sean