One more thing to consider besides the machine and frame is your own vision. I have a 17" machine and quite honestly, at my age, I would not have the ability to see a greater distance across the quilt top if my machine were longer. It can be pretty tough to see thin pencil lines or small pock marks placed to show you where to go, especially on busy fabrics. You would have the option to roll the quilt back and forth so you can see better but honestly, depending on your frame rolling the quilt can be a big job in it's self.
Another thing, besides considering if you have room for the frame and machine with good walking room around it is to consider the lighting of the area where you plan to place it at. Stand in the area at different times of the day and night (yes, sometimes we get out of bed at night to quilt) to see how much lighting there really is and try to see if you'll be able to see tiny pencil lines on busy fabric. Hiring an electrician to wire, re-wire or add more wiring to install more lights can be a pretty hefty unplanned expense.
I would also consider your own patience. There is a learning curve with every machine and I have heard too many quilters say that they haven't touched their machine since they got it __x_ years ago. Really! It takes a lot of practice and a lot of patience with yourself to get good at it.
Best wishes in making your decision!