I have the Mighty Press lite - by Stahls - it's 11 x 15 and PERFECT! So far I have not run into any limitations with the size of the press (my goodness it's only 4'' narrower on one side than a 15x15) but I get the lifetime warranty, great wattage, and a well built press.
I measured a bunch of the images on my husband's XXL shirts and not one went over 11x15 (not even close) and since my focus was youth, I knew I was golden because I could so XXL shirts, too. And I have.
Now if someone is doing really large print, sometimes the 16x20's do not even cover them somehow.... so at that point, to debate 11x15 or 15x15 would be mute to me.
I've heard good things about the presses you're looking at and know good things about mine. Both have their strong points, though they might be a little different in some ways.
Good luck. You'll be happy you pitched the iron. That's how I started, too. Was Wonderfullllll to just set up the transfer and lower a press lid for 30 seconds. Whatta joy!!