Well, you kind of answered your own question. If you are looking for redundancy there are benefits to both technologies.
T1 if from the same carrier will give you some redundancy, however, in most cases if one T1 goes out you will most likely lost both T1s. If staying with the same carrier is a needed then I would recommend that you have them put these Ts in different routers and have them diversified at different COs.
On the other hand you could order a different T1 from one carrier and one from another. Thus redundancy is mostly assured except where it enters your building and where it enters the same manhole facility.
ISDN is good for redundancy and for light email traffic. Not grand if you are pumping a 1544mb of data.
Using ISDN is a failsafe for 90% of cicumstantances since it's a dail and connect type of technology (not dedicated).
BGP is a protocol for setting up diversified internet connections to the same isp or mostly different isps. I would recommend that if you are using the same isp and not connected to the same router to use bgp. If you are connected to the same router and isp then loadbalance using two static routes point to your upstream router.
Good luck and i hope this helps.
