The part you're paying for isn't necessarily the uptime, it's the mean time to repair. You'd be surprised how long even a "five nines" uptime can be down when that's averaged out over a year. If you cable line does go down, how long do they typically take to fix it? What is the time stated in the leased line SLA for repair?
An example here in the states, a cable line could be down for days depending on how busy the cable co is and how much yelling is done. A leased line is typically repaired in less than 4 hours, but in either case it depends on the nature of the problem. If someone cuts a line with an excavator it's typically going to be down longer than if it's a bad card or other easily solved issue.
If you can handle a bit of downtime in either case, then the extra cash for the fancy SLA may not be worth it. If you can get lines from different providers that enter your building from different wire paths that's even better for redundancy. If the telco provides both the leased line and the ADSL, then odds are if one goes down, they both go down, but if you have a line from cable and another over phone lines then odds are one will remain up.
And not that it's relevant in your case, but even on a leased line between two sites, you'd still want to encrypt the traffic. Best practice (and by some standards, a requirement) is to encrypt anything that leaves your location and the network you physically control. Even if the line is "private" it's still equipment that could be compromised, either unknowingly by a third party, or willingly as in a telco granting access to a government agency.
