I agree with Neil, if you are looking at the redundancy features of Nortel's SMLT and Cisco's rapid Spanning tree, you will see that Nortel's setup is a greater advantage. Not to mention that in a SMLT/MLT, all links are used. Where as we all know in SPT, you lose one of the links.
As far as support goes, I hope anyone running with the 8600s or 6500's (Enterpirse Chassis's), has a support contract. If you are willing to spend that much money on a switch you atleast have a contract to boot as well. But my personal experience of the Tech support from both companies, Nortel has Cisco beat.
Personally, I look at Nortel as the advantage on the Switches. They have been developing the L3 switches alot longer than Cisco. Nortel, when it was Bay Networks, had purchased the technologies from Rapid City who built the first Accelar Switch. Rumor has it, it was proposed to Cisco, but they turned it down thinking who wants a L3 switch. It only routes IP. Bay Networks, took the box and developed it further. And in essence Nortel has had the avantage of a L3 switch. Look at the companies that have proposed IEEE Standards in L3 Switches, and you will see alot of the technologies are from Bay Networks. Food for thought.
Now if you were asking Routers... I would say Cisco. Cisco has always developed a better router.