We have both where I work. The T1 is used for our outward facing services. FTP, WWW, VPN, etc. The Comcast connection is used for all the out going requests. So people in the office web browsing etc. In the case the Comcast connection goes down this traffic falls back to the T1 as low priority traffic.
So both links are monitored and the cable connection does go down more often but this is normally in the middle of the night for a few minutes max. I'm assuming this is probably a maintenance window of some sort that Comcast uses to update things.
Bandwidth wise the main difference is the T1 is guaranteed where the Cable connection may be slow during peak times. In reality I haven't seen our Comcast connection slow down much and consistently see 16Mbit download speeds. The other difference you'll likely find is in the case of a disruption in service. The T1 provider will have more pressure on them to resolve the situation quickly due to the SLA. We had an ice storm here in the north east last year. The office was without power for 1 week but we were running on generators. The T1 surprisingly never went down. But it took about 3 days for Comcast to get their networks back up.
Security wise neither are secure and if you need security you should be using VPN's or encryption period. Anyone can sit on a telephone pole and clip a couple of wires to your circuit and capture all the data flowing on it.
