For art quilts, I think the sit down quilting machines might work better for you than the traditional short/mid/longarm on a frame.
Look at the APQS George (
http://www.apqs.com/machines_george.php), Handiquilter HQ 16 sitdown (
http://www.handiquilter.com/shop/pro...teen-sit-down/), The Queen Quilter 18 (
http://queenquilter18.com/, or take one of the longer throat machines like a Voyager, Bailey, Innova, etc. and set it into your own table. All of the above also sell the traditional quilting machines with frames as well as To Be Quilting (
http://www.tobequilting.com), Homesteader (
http://www.thequiltingsolution.com/), Nolting (
http://www.nolting.com), ABM (
http://www.abminternational.com/inno...ng-machine.php), Gammil (
http://www.gammill.com/), KenQuilt (
http://www.kenquilt.com/), Crystal Quilter from New Joy (
http://www.newjoyquilting.com/), Viking/Husqvarna MegaQuilter 18 x 8 (
http://www.husqvarnaviking.com/us/14755.htm) --this one is manufactured by Tin Lizzie, I believe and is also available from Pfaff as something else. Babylock also has one that is made by HandiQuilter.
I know there are more brands out there like A1 Elite, NuStyle, Prodigy, Pennywinkle Valley Ranch, etc. and there are people that take a regular 9" straight stitch machine like the Brother PQ1500, Janome MC1600p, Babylock QCP, Juki TL98 & stretch them into an 18" quilting machine--Wowquilts is the first one to pop into my head but others have done it.
How much space you have & how much your wallet can afford will be the major factors. Then you need to test drive as many different ones as you can to find one that fits you. There is no "one best brand" or most cost effective. They are as unique as the quilts loaded on them.