If you are just looking at overall speed then yes comcast or any other cable provider seems like a better deal. But with anything in the IT industry there are many other things you need to take into account. Most of the time a T1 will come with an SLA and IMO better support than comcast is going to give. T1's can be used for phone, data, or both. T1s offer options for managed routers for remote offices etc without a network technician. If you are a medium to large business you can choose the option for an MPLS T1 which allows you LAN like access to your equipment. Once it is configured properly there is no need for site to site tunneling etc.
All of these things and more must be taken into account when deciding between cable or a T1.