This is a bit of a tricky question to answer but generally I would say you will notice an improvement on some channels since a lot of time you will see random interference on some channels. On digital cable they will all be equally clear, if you do have trouble with your cable connection you may see some pixelation/freezing but I have not had trouble with mine.
I would recommend you spend the extra and get an HD box because even if you don't have an HDTV (which I see you do) you still get a few channels that are in HD (the 500+ channels). This can be viewed via the Coax, Composite or S-Video connections if you have an SDTV. This is a good way to future proof IMHO (especially if you decide to purchase the digital terminal).
I recommend setting the, "Pass through" setting on your digital terminal as in my experience the up conversion that the box does is worse (introduces some artifacting etc.) compared to the up conversion circuit on my TV.
I would reccomend digital cable also because of the Interactive Program Guide (IPG). I love being able to see what is on (or what I am watching for that matter) right on my TV, it's reason enough to get Digital Cable.
If you have enough money, I'd also recommend a PVR. Really cool to be able to pause live TV or just line up a bunch of shows that you like to watch, so you can sit down on the couch any time and enjoy what you want to watch instead of whatever junk is on (infomercials anyone? LOL).
I just got my 8300HD terminal today and it has pretty much every output known to man on it... Coaxial, Composite, S-Video, Component, HDMI video connections, Analog, Digital Coax, Optical audio outputs. So far it seems to be faster than the other Rogers Digital Terminals that I have used (the interface and IPG feels snappier, changes channels a bit quicker as well compared to my Explorer 3250HD).
So basically, yes I would recommend going to Digital Cable. Personally I'd like to see the shutdown of Analog cable to make more room for HDTV channels but sadly we are somewhat behind the US on that. I think I heard something like 2013 and even then Dependant on a certain percentage already being converted to Digital Cable, so who knows.