I have both in Sportsters and 3 other 17hmrs in bolt guns, the 17M2 is cheaper to shoot at $4-$6 a box, but still not as cheap as a 22Lr, but every bit as accurate and flat shooting as the 17hmr within its respective range, the 17M2 to about 100-125yds, where the 17hmr is good well past that, same bullets, 17gr @ ~2000fps for the M2, compared to ~2600fps for the Hmr. The Hmr has more bullet options too, Vmax, TNT hollow points and FMJ, in 17gr and 20gr in a couple flavors. If you want cheaper to shoot and slightly limited range, go for the M2, if you want a bit more power and range and don't mind $11-$12 a box for ammo, go for the Hmr, or get both and use the one you want, when ya want.
Complete rifles can be had for as little as $110-$140, not much more than an accessory barrel for $98.
Tim