MHP2G is just MHP2+extra content. It's really just an expansion pack where MHP2 players could transfer their character, and new players can just start from the beginning of MHP2.
That's not quite true, it's not just a simple expansion (ie: more monsters, weapons, armor, missions, etc) but there's lots of remixed content (new monster placement/drops, rebalanced weapons/armor, changed missions, etc). It's almost like comparing Metal Slug 2 to Metal Slug X.
'I still think there is a big advantage to put big titles on Wii instead of PS3 in Japan because they sell very well with the big userbase on Wii (see MH3 and DQX will also sell huge no doubt) and the development costs are lower than for the HDconsoles.'.
Conclusive proof right there!
Two of the biggest IPs in Japan, and one of those games not even released yet.
Dragon Quest is essentially mainstream, it pretty much demands a 10m+ userbase to sell in to. Larger IPs do tend to inherently benefit from larger insalled bases.
Also, DQ Swords released on Wii and it's the best selling (console) DQ spinoff in history in Japan.