Try the III before you make any decisions!
I tried them both (combo's) and because I preffered the SOUND of the III, I bought it. I played them both...over and over. The Iv's resale value and versatility were my initial main issues too. After falling in love with the TONE of the III, I gave up on the IV (no offense). I liked the IV's tone. They are very similar. But I was able to get a deeper, richer bottom on the III that no matter what I dialed on the IV, I couldn't match. The III's overdrive was also a little looser, not as compressed. Single notes especially, seemed to jump out nicer. All factors which enhance MY playing. Gain wise The III had more, but really not that much more. The IV was quieter to me. The III can be a pain to setup right. 2 of its 3 channels are easier to set up. Trying to set the "shared" Clean, R2, Lead buttons/sliders settings to all work together, can take a lot of tweaking. Plus, volume wise, when switching from the clean and R2 channels the Clean is louder (easy fix...if don't mind modding it). $1200CDN is about 1070ish USD, which is pretty normal for eBay Mark III's (IF they have the EQ, Reverb & Simulclass). I don't believe you can make a bad choice with either. But I would definately try them BOTH out. I am in NO WAY putting down the IV...it's on my list of dream amps!!