Warmers produce heat, then they have to transfer that heat to the towels. So a starting point is to compare watts or BTUs from one model to another to ascertain heat output. Output-wise, the more BTUs the heater can generate the faster it'll be able to heat the towels. The more surface area that is in contact with the towel, the faster the heat will be transferred. If you want to leave it on 24/7 then a low-BTU model might suffice, or a model with round rails might provide adequate heat transfer to the towels. If you want it on a programmed timer or if you just turn it on when you get into the shower, or if you need to batch heat towels one after another, then a high-BTU model with flat rails might be better. A nice touch is that the higher BTU models can serve as room warmers too. Personally if you want a true towel warmer I'm a fan of Runtal or Myson. From the towel warmer perspective the Runtal Omnipanel is a great warmer. Myson has a similar design with their Interlude series. I'm not familiar with Amba. My experiences with Mr. Steam and Warmrails is that they are fairly low-BTU units and thus suited for more casual use than serious warming. They work mind you, and they have their fans. Nothing wrong with that. But they are not as versatile as the higher BTU units. If you see some outputs listed in watts and others in BTUs, multiply watts by 3.41 to get BTUs. Example, 100 watts equates to 341 BTUs.

We just installed a Runtal Omnipanel in a bathroom. The 'bars' on this one are flat so there is a lot of surface area to contact the towels. There are various sizes and we got one about 24" wide and nearly 40" tall, it has 3 gaps to stick towels through to hang on it. It has 3 temperatures, low, medium, and high and it gets very hot on high. I don't think it would burn a person on contact, but its too hot to place your hand on for long. And...the towels dry up fabulously on it! My kids turn it on, heats up in about 20 minutes, take their shower, dry off with a nice warm towel, and then hang it back up to dry. They can turn the unit off at that point, and the towels dry up great on the residual heat. The units are very solid, its like a cast iron feel; Runtal makes a variety of designs and sizes, and colors as well as chrome finishes. They do direct wire, plug-in and hydronic units. We are very pleased with this.

The UK ones like this are connected to the central heating or the boiler which is why they have pipes coming from the floor and are valved like this. Most America towel heaters are electric.

My bathroom reno is getting a hydronic (not electric) towel warmer inside the shower. It is replacing the hot-water baseboard heater currently outside the shower. I already know that the towel won't get wet since I've been hanging my towel on a hook in the shower for some years so I don't have to open the slider and let the warmth out until I'm dry. When I get out of the shower, I hang my towel to dry on a hook outside the shower. The system works well for me. For me, the main benefit of the hydronic towel warmer is not that it warms towels but that it's rated to warm the whole room. (I loathe baseboard heaters.) The warm towel is just an added bonus.