Dubya, I'm probably the only person here who owns Polks and Axioms. I have a 5.1 Polk system with RTi8 floorstanders and an Axiom stereo system with M22s. The Monitor 70s replaced the RTi8s as a lower budget option at the local Circuit City about a year after I bought the Polk system and I have heard them. They did not impress me as much as the RTi8s (in the same room, but over a year later and I have no idea what other equipment was in use). After giving this a lot of thought and comparing the speakers in my home on numerous occasions I have come to some conclusions - take them for what they are worth.
Polk is a good company that makes good speakers. Their downside (or upside depending on how you look at it) is that they try to have a variety of lines at different price points so that they can appeal to a broad market. As a result, they don't seem to put as much effort as Axiom into the details of each model that they make. Their other downside is that they don't sell direct. Overall, my general feel is that Axiom gives you 20-40% more for your money when you compare Polk products at their retail prices. However, the price you're describing for Monitor 70s is a good deal.
So, my advice is to base your decision on room size and how loud you like to play the music - and maybe how likely you are to move to a bigger room in the future. If the M3s will give you the volume you want you will probably like them better. If not, the Monitor 70s might be a better choice for you.
Of course, the advice you already got - "try them out side-by-side and give us a good report" is the best advice.