Eligible for the 72-hour visa-free transit?

Kiyuk, it is 144 hour visa free transit in Guangzhou and not 72 hour visa free transit. In other words, you can stay for up to 6 days in the entire Guangdong province - visa free. So, your both itineraries are perfectly within the 144 hour visa free transit rules.

If you decide to take a train from Guangzhou to Hong Kong, that is even better than taking a flight because it is cheaper and also more convenient on so short distance, you will have to buy your train ticket in advance, even before your departure from Canada and print it out. You will have that printout at hand, ready to be presented to relevant authorities as a proof your transit intention. Also, after your stay in Guangzhou and in general in Guangdong province, you will have to take exactly that train you have bought your ticket for. Please, do not plan to take a bus because that would involve several complicated details because it will be hard to find a through bus and a ticket that you could buy in advance. Taking a train is the way to go.

In the end, I am a bit worried about your itinerary because, as far as I know, there are no direct flights between Vancouver and Guangzhou. Consequently, you cannot fly on a nonstop flight from Vancouver to Guangzhou because a transit point is required. It is fine to have a transit point, but it has to be in a third country (for example in South Korea) and it cannot be anywhere in mainland China! The basic rule of 72/144 hour visa free transit clearly states that the immediate points before and after your mainland Chinese stop (Guangzhou in your case), have to be in two different countries or eligible territories (eligible territories are Hong Kong, Taiwan and Macau). In any case, I wish you a nice journey.