I think the idea of having an adjustable table is just a selling gimmick...it's easier for them to ship that way and so they sell you on the idea that it is better. Most people, once they have it set up, leave it set up all the time. (The back of the table is an excellent extra work table when your are not quilting.) For the best quilting results the table should be level, the bars should be accurately parallel, and the tracks should be straight and continuous with no "cuts" or "joints" to create a "hiccup" in the stitching when quilting. Poles that can be made smaller or larger, are by nature less rigid and can give or bend when tension is put on the quilt sandwich, possibly creating an "hourglass" shaped quilt.
Where do you store it when it is taken apart? Who wants to lift that heavy machine on and off the table? You CAN do a small quilt on a large frame or even load two small quilts at the same time onto one backing.
Shop around and don't make price your primary consideration. The time you spend trying to make a less than perfect set up work is worth something too! It should be fun. Many of the longarm companies will provide a custom size table length for a minimual charge if space is an issue.
Narrow your choices down to two or three options then ask to see a machine setup at full length. The small "demo" tables stitch pretty good...but do they also stitch as smoothly on a full length table setup?
Also ask about training, service, support, and warranties.