I haven't ridden the Infinity, but the Twin Sister is an awesome ride...super smooth, damp, incredibly stable, but it's still easy to turn and just a really nice ride. There is barely any camber to it-it's pretty flat in the middle. I had no worries about catching edges while on it.
I would think the 149 would be big enough for you, especially if you are concerned about maneuverability, unless you are lucky enough to be in a lot of powder-then you'd probably prefer the extra bit of length. It's not particularly stiff, but it's definitely not a noodle. It's a great all mountain board and the mellow magnetraction definitely gives it grip on ice, without making it grabby.
The Twin Sister and Roxy Ollie Pop were my two favorite boards from last year's demos...TOTALLY different though...the Ollie Pop is a park board. It handles everything on the mountain, but is much softer and not quite as damp. The OP has rocker between the binders and camber outside which is the opposite of the Twin Sister so it's more playful, but not as smooth of a ride. I don't hang out in the park, but I tend to prefer a more playful board. For the type of riding you describe, the Twin Sister sounds like a great fit.