ON to PEI - Through the US or...?

we do it a little different on the food. Bring food into Canada that you like. In Canada it will cost a lot more.
Just understand what you can bring in.
https://www.tripsavvy.com/what-can-i-bring-to-canada-1481670

To be safe we always say, when asked: 'we don't think we have anything...but lets check' it'll keep you from get fined.

and both routes are fine.

We have travelled both routes over the years and aside from some savings due to gas prices, both are fine. To me it comes down to where we want to stop and what sightseeing we want to do along the way.

There aren't any huge inclines or major elevation changes on the primary route you show. There are a few more along the grayed out route, but nothing too rough in my opinion. If you go a little farther north in the US, through the Adirondacks and across Vermont and New Hampshire, you do run into a good bit more hilly terrain, though I don't know if the overall fuel consumption is really all that much higher since your average speed is lower. I generally prefer it due to the pretty scenery and more moderate traffic.

Your blue route is taking you along the New York Thruway, the Mass Pike, and the Maine Turnpike, all of which are toll roads. The Thruway, in particular seems awfully expensive when traveling it; I think that's partly because it's a pretty long road, and the tolls add up. Tolls may eat up most or all of your potential fuel savings.

The Canadian route is pretty but somewhat longer and slower.

I have never really experienced extraordinarily long or hard border crossings, but I suppose there's always a first time. That doesn't mean it's always instantaneous.

We stick to Ontario when we travel from Winnipeg, with boarder wait times, tolls etc we don't save enough in fuel to justify it.

Keep in mind, best to travel through the boarder with a near empty fridge, call Canada and USA customs and find out what you can travel with. We prefer to simply say "the fridge and freezer are empty"

Did I mention that the drive through Ontario is amazing?