The Q50 is one of the smaller cars on your list but perhaps the most sporty. You can spend between $45k and almost $60k depending on which version/options you get. The q50S Hybrid is the most costly. This is the first year for the Q50 so it is commanding a higher price relative to invoice than many of the others.

The Q70 (the old M37 /M56) is a bigger car more along the lines of a 5series / EClass. It is more roomy than the Q50 if that's what you need. But it's a little less sporty, even with S trim. But they are a better deal because many dealerships are still only selling 2013 Ms and don't yet have the re-badged Q70. You can save thousands off of invoice (much more than a Q50 discount). The downside is that just last week infiniti released photos of the refreshed Q70: new front and back lights, a new q50-like grill. So your new M will look aged in less than a year if you care about that.

I own 2 Infinitis and love the brand but most people I know go German. As a side: Hyundai gives you a great car for the money if you aren't too snobby to drive one