The 2100 with keypad is cheaper, but I found it a lot more difficult to navigate through the menus and enter data, particularly if you are on the go. I started with 2100, but after a couple days changed it for a 2600, which has been trouble free for 5 years. If doing it again, I wouldn't consider the 2100, but I do a lot of changing AB lines, and that might just be me. The extra convenience of the touchscreen far outweighs the cost.
I'd reccomend that you get your dealer to demo them together, and see what works best for you.
I've read somewhere here that with the updates a 2600 will do everything a 2630 does except the remote camera view. I'm guessing the 2100 should be the same?