Need help choosing a HRM

I would prefer a HRM that is just a watch, they are not as expensive as the ones with straps. But my question is, are they just as reliable? They watch only ones that I have looked at just have you press one of your fingers on the metal plate on the back of it and you get a reading for calorie burn that way. Im assuming one with a chest strap would be a better option since it's tracking your heart the entire time.. yes? No? Id like to stay under $30 for this and will be stalking amazon/ebay for items as well. What HRM do you use and why did you pick that one?