the choice between the 12000 and the 15000 is hands down in favor of the 15000.
the 15000 "replaced" the 12000, so is currently at the top of the line.
manufacturer support (such as software updates, etc) will be available further into the future for the 15000.
both machines are the same size and will handle up to a king size quilt.
the key to embroidery-quilting large quilts - no matter which brand or model machine - is to keep the weight of the quilt off the hoop. that takes practice. check out this post to see how one member made her own frame for doing that:
PVC Embroidery/Quilt support frame