There are pros and cons to both sites.
As a seller, I prefer ebay.ca because it allows me to list in CAD and offer real actual calculated postage costs to my buyers. It also has a fully functional Shopping Cart. If I am buying from a Canadian seller, I also use ebay.ca to avoid USD-listed items and conversion rates. (The price in dollars in italics are USD-listed while CAD is straight boldface.)
As a buyer, I often use ebay.com to accrue the Bucks to buy from Americans since I also have an American shipping address. HOWEVER, the Shopping Cart on ebay.com as well as Shopping Basket ebay.co.uk cannot accommodate items where the listings were created outside of those sites. So, combined shipping can be a problem. (Remember, if you are browsing for goods to buy in ebay.com you will see the USD in boldface then and maybe CAD in italics.)
Which one to use? It really depends on what you want and need at that given time.