I'd go with the long arm if you already have the sewing machine that you need.
With the longarm you can quilt larger quilts, load several smaller ones at the same time as long as they all have the same backing on them.
It also depends on the longarm you are looking at in comparison and what bells and whistles it has.
I have a Nolting 18" Longarm and love it! I also have the computer system set up with it which makes it really nice!
I have a less expensive Brother SQ9000 sewing machine that I also love. It does what I need it too.
If you have questions feel free to ask.