So I would recommend being published in your home country in general if your work works for your home country's market because there is a lot of promo these days, especially with kidlit and school visits and such. And you want to be available for that and not have to cross borders.
Generally too if you are going to choose between the USA and Canadian markets and you are American, going with the USA market makes way more sense as it is MUCH bigger. Canada has a population that is slightly smaller than California. In the States comes more opportunity for bigger fan base, more money, and bigger success. Fame in Canada does not often translate to fame anywhere else (with the rare exceptions of the Margaret Atwoods). Whereas American fame tends to be international fame too.
That being said there are a couple advantages to publishing specifically picture books in Canada especially if you are Canadian. First of all, the kidlit support system here is amazing. And Canadians love Canadian kidlit. Second since the market is smaller, you have more of a chance of publishing smaller books. I'm not sure what your picture book is about, but picture books are like any books: there are trends, there are what people are looking for, there are issues that are particularly topical. There are "big books". I recently was discussing with my American agent about a "small book" idea for a picture book and said I thought it made more sense for me to submit on my own to some of the smaller Canadian kidlit publishers here and she agreed. That these days pitching a picture book in the States requires "big book" ideas.
So publishing in Canada absolutely has that advantage. That being said the advance will likely be small, the number of copies printed not that much either (a bestseller in Canada is around 5000 copies sold - so . . . I mean . . . that's small).
Another advantage to being Canadian and published in Canada is that Canadian publishers LOVE promoting their Canadian authors. There are also so many promotional opportunities that exist here in Canada for kidlit authors. It's pretty awesome.
So in general, I would stick with aiming to get an agent locally and then hope that your agent is able to sell in multiple markets (which happens a lot). Your Canadian friend . . . depending on how "big" the book is, there is an advantage to them staying local. But if they think they can have a big trendy book, then I would also recommend an American agent who then sells back to Canada later (preferably a separate sale, not a distribution sale where the Americans agree to publish the book and also will send it up to Canada - generally it's better to be published not distributed in your home country where you will promote because publishers tend to be more interested in promoting books they publish more than distribute. That being said, I've been quite lucky in that I've had some excellent support from publishers distributing me, so it's not all or nothing.)
As for your concerns about reviews: yes, if you are solely being published by a Canadian publisher you will only be reviewed by Canadian review publications. It makes no sense to otherwise as your book won't be available for purchase in the States (yes online they can them, but that doesn't count it seems to reviewers. The point is, it won't be on the shelves in any American bookstores). Remember, Canada is a completely separate country from the USA. Getting published only here means you are only published here. It doesn't somehow trickle down to the States.
That's my long answer.
Here's my brutal answer:
You're overthinking this. Get an American agent. You live in the States, America is a huge market, American agents are some of the most sought after in the world. They will sell your work to foreign markets.
BUT.
If your picture book isn't a "big book" you might have difficulty landing an agent at all with this one. Picture books are notoriously hard to get agents for. So if that is the case don't get upset or defeated. Picture books are also some of the only books an author can sell to big publishers on their own, without an agent.
And there you go! My answer
. Hope it was helpful!