Zilla thermostat and other cheap thermostat that are alike are prone to fluctuation and failure, the problem is when they fail rather than cutting the heat off they simply no longer regulate it. (been there done that back)

Given how many years your animal will live if you want a piece of mind what you should get is a reliable digital thermostat that will not allow fluctuation and that will not be prone to failure, well worth the little extra money.

You can go with Ranco, Johnson on the low end and Herpstat and Helix on the high end.