I like to use united.ca to check availability only.
My experience is that the base fare is the same on each countries United site as they all use the same databases. But the way they display the fares, base, and final is different for each country.

There would have to be some BIG MP bonuses for me to buy my tickets online again. Pray to God you don't need a refund, if you do buy onlline. The exchange rates cause a trail of confusion and debris that can (and has ) taken them months to sort out.

Sad but true, I recommend calling the 800#.

Use the cheapest routing you were able to find online and croos your fingers for the same fare...