Only buy cigars from the La Casa de Habanos (LCDH) and airport shop. You are allowed to bring back to Canada 50 sticks per person. If you are buying a box or more, you must show a proper receipt to the Cubans on the way out. You will not get the proper receipt from the guy on the street or his buddy at the resort. No receipt? The cigars don't go with you! You will only get the receipt from the LCDH.
From my research, the prices of cigars in Cuba are not that much of a savings over the Canadian internet retailer that I use.
As far as what to buy, try different singles while you are there and then make your purchase on the way home. The Monte #4 is okay but it is kind of like the basic Cuban cigar that everyone knows. Monte #2 are great. The Jose De Piedro are fantastic little hand finished cigars that are really well priced.
Remember, if you don't buy at LCDH you may be getting fakes too. There was never, ever, a glass-top box of Cohiba's ever produced in Cuba. You'd be surprised how many have bought one though.