Which Juki do you have and if it's the TL98, why would you need another very similar machine? To me it sounds like you are buying another machine very similar to one you all ready own so unless you are physically unable to move it from frame to table why would you shell out $500?
It also sounds like the other, more expensive machine may have been a lot closer to a "longarm" than the $500 one you are about to purchase. Viking has 2 machines they call "Mega Quilters". One is very similar to a Juki TL98/TL2000 and new, retail price is usually between $750 & $900--maybe more if they package it with some extra goodies. The other Mega Quilter is the 18.8 or 18 x 8 which is a Tin Lizzie 18" industrial looking beastie made for Viking and now Janome. It may be sold under other "brands" as well.