I consider the M2 and the M3 interchangeable, six of one - half dozen of the other. It has a slightly different tread design for higher speed resistance to hydroplaning on standing water. It was originally released only in hard-to-find sizes but is gradually replacing the M2 in 16, 17, and 18" sizes. The M2 is available in 14, 15, 16, and 17" sizes.

I use the M2s on stock wheels for 3-4 mos. per year (1200mi/month) and am expecting 3 seasons out of them. My wagon was an absolute demon in the snow last year - never felt so sure-footed in the most adverse of conditions.

Bottom line, in M2 vs. M3, I'd go either with whatever one is less expensive and/or in-stock in your size.

The tires are $137 - $139 each and shipping on a set of four will be around $38 total.