I have a midarm on a frame, but mine is a Voyager with a 17" throat. The 17" is really nice to have. I would categorize the Brother 1500S as a shortarm machine rather than a midarm. Midarms start at about a 13" harp. In my opinion, anything less than 13" (or maybe the 12" that Suzie mentions) is going to be very limiting.
A lot depends too on what frame you are considering. The Ken Lund frame is very inexpensive compared to others and could certainly be worth a try. Here is his website (you can buy a kit from him).
http://machine-quilt-frames.blogspot.com/
He has quite a few videos on Youtube also showing how he builds and uses his frame. If you use the "search" feature in the upper righthand corner of the Quilting Board, you will find some very informative threads about the Ken Lund frame from people who are using it.
I do not think investing in a more expensive frame is worth it unless you have a machine with a bigger harp than the Brother 1500.
My setup (Voyager 17 with Hinterberg frame) was still available from the Hinterberg website for about $4,000 last time I looked. The Voyager is no longer made (it is a stretched industrial Singer) but they still refurbish Voyagers for resale. I bought my setup for less than that from a quilter in my guild who wanted to upgrade. There are websites that list used setups too. Another QB member found one listed in her area that way.