It's all going to depend on the featuers you have configured, the amount of memory in each one, etc. There's no "magical" difference between a "layer 3 switch" and a "router," they both do the same thing. "Layer 3 switch" is a marketing term--in the real world, there are bridges, and there are routers. Any time you rewrite the mac header on a packet, you are routing. :-)
So, don't think of it in terms of these being two different types of devices, one of which is inherintly "superior" to the other in some way, but simply as two routers. Consider the price/performance tradeoff, etc.
In this case, will both of them handle the number of routes you intend to throw at them? If the link speed increases, what are your options for changing out the interface type without throwing the box away, or is it better in either case to replace the box? What's the max througput on the box, and on what sorts of interfaces (the max throughput on a given box might be really high on ethernet interfaces, and much lower on serial interfaces, etc).
Another consideration is the feature set you intend to enable. Access lists? What impact do they have on performance? BGP convergence speed? How much total memory can the box handle? Etc....
It's not so simple as it looks at first. In your case, I think either one will do the job, and you probably won't see any problems with the switching speed on a 2600. At least not on a T1. Ethernet to ethernet speeds on a 3550 are certainly faster, and port density is higher, but these may not be requirements in your case. Can the 3550 run IOS firwall, or do you intend to put in a seperate box for that? NAT?
Again, it's hard to know without looking at all of the things you'd like to address with the box.
Russ.W
