Look at the size of the blades you produce.....then make your decision based on the largest you ever expect to make. Not to step on Doug's toes, but I've owned both 110v and 220v ovens....and would only consider purchasing/using a 220v model (that is IF you have 220v in your shop, or can install it)
Personally, I have two heat treat ovens, both are 18" long interiors......it's was just how things worked out that allowed me to have two, but I have discovered a great advantage in having two......you can austinize in one, and temper in the other, without having to wait the hours that it normally takes an oven to cool down to tempering temp range.
Finally, I've owned and used several different brands, and if at all possible, spend the money and get a Paragon. I used to think there were little or no differences between brands, but have come to realize that the Paragon ovens are simply overall better ovens then the other brands.