The i250 is inexpensive, but it is also the most limited of all the Canon printers, in the sense that it cannot produce a bleed and will not print unless the paper tray is attached. If that doesn't bother you, you can happily go for it.
The i350 is just like the i250 but a little faster when printing from the paper tray and without a margin. However, we wouldn't recommend it. If today, in mid-July, it seems to be well positioned, this won't be the case in August when the new HP range comes out. It will then be confronted with a competitor that is a much better printer in numerous respects. But it's too soon to talk about that.
So it's true. The i450 is a lighter, slower version of the i850, less expensive to buy, very good in photo mode and a little more expensive in terms of ink. So it's a very good choice for anyone who wants a low-priced printer for producing text and photos, though in small quantities.