When I used exo terras I had glass cut to fit the top then I drilled it and attached a radiant heat panel. It kept the hot spot beautifully hot and the cool side stayed where I needed it as well. If I were to do this again I'd probably skip having glass cut and just use a 1/4" thick piece of plywood or scrap pvc sheet.
Of your 2 options you mention, I'd be inclined to use the ultratherm as the heat cable I've used in the past had portions that ran considerably hotter than the rest of it.
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