I have the 15" on a grace pro frame. I wouldn't waste time/money on the 13".
it's a regular sewing machine (Janome) that has been stretched, very basic, no bells or whistles.
frame quilting takes some getting used to, not as easy as any of think it will be, so as with every other aspect there is a learning curve and it takes a LOT of practice
I have never regreted my purchase, they have come out with a ruler foot for it now so it's getting better all the time for me!
there is a yahoo group, we have discussed all the ina and outs we've had, you can read through there and maybe help ypu decide. I have no experience with any other machine, bought mine site unseen strictly because there is a woman on here names June who has one and she went on and on about it, she's good advertisement for them.