I've done it one way only, from Vancouver to Seattle, by train. I already had a visa waiver from earlier in my trip.

You go through US immigration before boarding the train in Vancouver. The guy looked at the waiver stub and my passport, asked how long I was staying in the US this time, and admitted me for the remainder of the original 90 days. Then just walk to the train, checking any large baggage.

The train sat in a locked yard until departure then crawled for about half an hour to the border. The system is supposed to be that the customs officials come aboard and check passengers while the train travels to its next stop. However they were short staffed or something so we had to wait at the border until they were finished. After that the journey to Seattle was a bit quicker - they proudly announced that the train was doing its maximum of 78mph at one stage.

Getting your checked baggage back at Seattle is a lot quicker than doing the same thing at any but the smallest airport.

Travelling by road all formalities are done at the border. Depends on how busy they are as to how long it takes - by car, off-peak, a few minutes only. By bus it can be problematical if the driver doesn't want to wait for the few passengers who are held up in immigration.

I'd do it by train if the times suit (morning out, evening back - there's only one a day), otherwise by car unless you hate the idea of driving.

If you are feeling adventurous you might consider getting the ferry to Vancouver Island, another to Vancouver and returning direct.

Main thing is on the way out DO NOT let the Canadian immigration guy take your visa waiver stub - if you still have it it will make getting back to the US a lot easier.