I've heard all three and in regards to Mike's comments, agree on the RTi series over the Monitor. The Polk Audio Monitor isn't my cup of tea, or beer, either. The new RTiA series sounds a bit more refined in regards to midrange response over the previous RTi.
It's unfortunate that you can't easily get a demo but that happens.
Out of those 3 brands, based on my ears and electronics....I'd probably go with the Mission brand. I prefer vintage Monitor Audio and the Polk RTi series is a bit too forward on my rig. I had a little pair of Mission MS50 that I loved and used for years on my computer rig. YMMV and good luck.
Mark
Polk Audio CS