Well, I do actually think we can draw from last gen, because it isn't all that different to this one. The 3DS is a more casual friendly handheld and the Vita is more for the core.
If Sony don't cut this year also, the 3DS is just going to grow a bigger lead.
When you think about it, 360 started with a 5m lead against PS3, it grew to 8, and now it's down to 3, the 3DS has a lead of 15m, that's going to grow pretty fast this year, and there is nothing but a price drop to suggest why it's going to start selling above 3DS in such large numbers in the future.
I do think think weekly or yearly marketshare will be more evenly split earlier this gen though, as oppose to the DS dominating the majority of its life.