What should I look for when comparing electric floor heating companies?

Look for confidence in the quality of the heaters they make. For ease of use, long term reliability and robustness - compare wire types. In particular ask if the wire in question is 2mm thick and coated on the inside and outside with a hard material like ETFE or a soft one like PVC. Warmup's wire is industry leading in all these areas and comes with a lifetime warranty. PVC based wires only come with a ten year guarantee - for a good reason.