I took the 250 for a test ride and it has plenty enough pep for me. My only concern with getting that (over the 400) would be resale value down the road as I'm an expat and uncertain how long I'll be here. My guess is that if I wanted to sell it in 2-3 years, I'd take a huge hit in depreciation, one, because the 400 will be out by then and most people will prefer that, two, because other brands may have 3-wheel scooters (Gilera, maybe the Japanese...) Moreover, this concept of 2 front wheels is new, so the overall price level may drop after a while, so that what costs 12000 today might be 10000 in three years.
But purely for myself, 400 cc would mean worse fuel economy, slightly more weight, perhaps smaller underseat storage space... Since i don't do much highway riding, everything more than 250 is overkill. Even on highways, I'm not sure if more than 250 is of any use either.