I only did freight because mid-2008 when I had the hours, everybody (121) was furloughing. I did 3+ years at Ameriflight until the economy recovered somewhat then I went 121.
Let me speak from experience: STAY AWAY FROM 135 FREIGHT. It is the easiest way to jack up your resume. For example, my new hire class consisted of 6 pilots. Over the 3 years, all were fired except me. They want you to fly broken planes, go below minimums to get into places. They treat you like a wuss if you refuse a broken airplane. If you squak a plane with a minor issue the other pilots will talk trash behind your back in the office (heard that stuff all the time). The hours suck. Most weeks I worked 6 days.
Management sucks if your are not buddy-buddy. I was asked by the director of training about my opinion of gays in the military (he is gay, and was thinking about making me a training captain). Equal opportunity? I think not. Oh yeah, and they will try to convince you that they will have a flow-up program with Southwest hahaha.
Take my advice: ABSOLUTELY DEFINITELY GO 121!!!! Do yourself a favor. The only thing I got out of Ameriflight is 1500 turbine pic (which will get me almost nowhere). 121 (even regional) is a thousand times better. Better pay (despite what AMF'rs tell you), way better schedule, way better management (in most cases). In my second year at a regional I made 15K more than I did my third year at AMF.
So other than that, I don't think it really matters where you go