Eheim canisters are probably the best ones available for long-term dependability. The classic series are the simplest and less expensive, the Pro II series are a bit fancier and easier to prime. The thermofilters have a rather expensive (to replace) heater inside the filter case, never had one of those but do have several of the Pro II filters. I got them used, had to replace the main pump head gasket on one because it had been sitting dry for many months.
If the heater doesn't work I wouldn't worry about it, just use a regular heater and forget it. That won't affect the capacity or operation of the rest of the filter either way.