I've been in the hobby seven years and have only used a trim iron once that belonged to a friend who taught me to cover. There will be a few spots where it would be handy, but for covering most sport planes you can do find without it. I'll second the advice to just get the one that's cheapest. The covering has a pretty wide temperature range to work it in so there's really no reason to sweat it if the bi-metal thermostat lets the shoe heat swing by 10 degrees.