I bought my longarm 9 years ago and never looked back. My main reason was ergonomic -- standing upright instead of sitting was much better for me. I was always hunched over when using my domestic for quilting, partly because my arms are long and my eyesight's not so great. I used a friend's mid-arm once, and it was still not comfortable for me.
Bigger machines take up lots more space, of course, but if you have room and can afford it, I'd say go for it.