I'm not familiar with the Thermics, but I've been very pleased with the Hotronics. I have the M4's in both my wife's and my boots, and they work great, keeping toes comfy all day long.
But what impresses me most about Hotronics is customer service. I bought ours last Spring on eBay but didn't check them out until this Fall. It was only then I discovered that one of the batteries wouldn't take a charge. Hotronics exchanged it, no questions asked.
As luck would have it, one of the chargers gave out earlier this month. This time, because it was in the middle of Winter and the customer service rep said she didn't want me to wait, Hotronics sent out a replacement by FedEx 2-Day delivery (on their dime) before I returned the broken one, and again, no questions asked.
Good product, great service, imho