Fellow Calgarian here :)
First, the Eurostar is simply wonderful. It's fast, comfortable and smooth as silk. It's definately not a C-Train ride. However, the chunnel is simply 20 minutes of dark, that's it. You never see the coast. It's just a tunnel. Unless your kids like trains, it's just a fast train ride with a nicer view on the France side. In my opinion, what makes the Eurostar so special is how seriously uneventful it is.
For advance ticket purchase, it really depends on your flexability. If you need to go on a specific date, time and fare class then book as early as possible. If you are flexible on time of day and don't mind lower fare classes, you can wait. Last August, my wife and I just went to the station (both in London and Paris), purchased tickets, had a nice slow meal and was on a train. But yes, the longer you wait, the more expensive it is.
Pete