Depends on the system. Normally the units sold by places like spectrapure and BRS just add a second carbon filter or if you are getting a unit to remove chloramines, they add a special filter that breaks down chloramines. The second carbon filter is just added protection from chlorine and other chemicals. If chlorine makes it past the carbon filter it will destroy the ro membrane. If you are a very cautious person, then a 5 stage may be for you. Realize, all these systems have the same basic generic hardware, so you can add a filter to the system at any time. My RO unit is over 25 years old. It started out as a 3 stage 20gpd unit back in the early 90's. It is now a 5 stage with a spectrapure 99% rejection rate membrane and a booster pump. I added a second carbon filter to mine because I'm lazy and I'm very bad about waiting more than the recommended 6 months to change the sediment and carbon filters.
The other "5" stage systems I've seen are usually not sold as a unit but put together after wards by buying the parts or as a separate kit. The 5th stage is a second di filter. In this setup, when you start seeing greater than 0 tds product water from the first di filter you remove it and move the second di filter into its spot. You then refill the di filter you removed and put it in the second spot. This way you are always getting 0 product water.
When buying your RO unit, you also need to get a tds meter. I personally like the handhelds but most prefer the convenience of the inline ones. Also, since you are in Canada, I'm assuming your tap water is fairly cold. Water temp plays a big part in the production rate. The gpd you see for these units is a rating at a temperature of I believe around 77. I've never tested the temp of my water but in the summer (north Texas) my 90 gpd unit fills my 5 gal jug in about 60 minutes. That same jug takes around 90 minutes right now to fill.