I use the "arm" type one for cutting things that don't need to be all that accurate or have that perfect of an edge. They are quick, and good enough for test strips and trimming paper down before printing. I use a rotary trimmer only to slice prints for dry mounting, once the dry mount tissue has been tacked onto the back. This saves the blade of the rotary trimmer for important stuff. So, I would get one of each, especially since the arm type are so cheap. It is easy to sharpen the arm type blade yourself as well, using a standard kitchen knife sharpener.
If I was not dry mounting, the arm type would be perfectly sufficient for me.