I think this is a very tough question and decision to make. But there's one category that acts as the kicker for me, and that's # of saves.
Last season, Lundqvist faced 2109 shots and made 1942 saves.
In 2008-09, Lundqvist faced 2007 shots and made 1839 saves.
Last season, Rask only faced 1221 shots and made 1137 saves. Obviously that's only in 45 games and Lundqvist played in 73.
For next season, you'll see Rask face a lot more shots total, but he still won't have the workload of Lundqvist. You'll need to take that into consideration while making your decision.
Yes, the new pad regulations will hurt Lundqvist in actual numbers. But I think after a few months, this will FINALLY force Lundqvist to start playing a step higher in his crease. He could get away with it wearing 5" thigh-rises, but with only 2 or 3-inch thigh-rises, he's going to get ripped apart if he continues to play on his goal line.
Now I'm not saying Lundqvist is guaranteed to start playing a step higher, but there's a very good chance he will start to do so. He has to, because it's the most logical and effective way to combat the loss of leg pad and net coverage. Geometrically and mathematically, it's bound to happen sooner or later.
Since you state that you are kind of worried about Rask pulling a Mason, why don't you take a sure thing and secure Lundqvist? The difference between Rask and Lundy are really splitting hairs from a fantasy perspective, but you know Lundqvist is going to play at least 65 games, whereas you don't have that same confident feeling with Rask.
As much as I love Rask's ability, you are not wrong to think that Rask could struggle this season. Plus he won't face as many shots on average as Lundqvist. So I would take Lundqvist in this situation!! You never know, maybe those smaller pads DO force him to take a step out, and after a few months of adjusting and getting into a rhythm, he's playing the best hockey of his life!!!!