—Mini would free up some desk space, if that matters to you.
—Mini is potentially cheaper, if budget is an important consideration. With the iMac you're paying for a display, which you probably don't need if you already have two externals. I can't imagine using three displays
—iMac has better specs, but if you're mainly doing word processing and web browsing, it may not matter a ton (spec comparison: http://www.apple.com/mac/compare/desktops.html).
I used a 15" MBP with a 30" Cinema Display at my last job and usually kept one frequently used app open in full screen mode on the MBP. The retina display is so nice, I found I preferred it to the 30" screen sometimes.