Switching always has the advantage of new signup bonuses!!!
I have both Chase and Amex, and one thing I like about Chase is that points are transferrable. I buy points from co-workers and have told family that points would make a great Christmas present. I don't think Amex has rules against buying or selling points, but they are not transferrable, meaning that if I buy them, I have to deposit them immediately in a travel partner account.
Mostly it depends which partners you want to use. Amex has more, but you should figure out which one(s) you'd want to use.
Also, I think Chase points can be more flexible. Unless I'm mistaken, UR points can be turned into cash for 1 cent or into pretty much any kind of travel for 1.25 cents. I think Amex points cannot be cashed, can be turned into gift cards for under 1 cent, and you can get 1.25 cents for travel only with Membership Rewards First, which requires a plat card.
To me personally, Amex points are most valuable, because I need Air Canada points the most. But both have their value.
If you get an Amex card, there are a number of options. The Plat card has a very high fee ($450, not waived first year) but the best signup bonus and a lot of other really good benefits. Preferred Rewards Gold has 25K signup and 3X on airfare, 2X on gas and groceries, and 15K bonus if you spend $30K in a year. Those are usually the two best options.