I'm in Toronto, where there's virtually no competition; you get cable from Rogers, and consumer DSL from Bell. I've always had cable, and I've been lucky to be in neighbourhoods where connection speeds have been consistently high, with virtually no downtime ever. Generally, I tell my friends to try cable first, because it's cheaper and has the potential to be faster than the crappy ADSL implementation that Bell installs. Your results will vary. If you don't use your ISP for your e-mail, you have the flexibility to switch providers at any time. Spending a couple of bucks to register your own domain and get virtual e-mail hosting has proven to be money well spent for me.
Finally, as to your splitter question: I've always split my cable modem signal to connect to my AIW Radeon on my work computer, and I've never had any problems doing so. I've always just used a standard splitter, too. Do not use any type of signal amplifier on your cable modem's line. You'll have no connectivity if you do.
Your cable company is probably wise to the fact that you'll end up running a splitter to another TV from your cable modem line. So, don't tell them that's what you're doing, let the installer do his job, connect your modem, and leave. If your provider allows a self-install option (I'd never let any of their inexperienced people so much as touch any of my computers), go that route. It's often cheaper, and they won't install a pile of useless spyware on your computer, as Rogers does.