By "swift" you mean 'fast'?
Flash memory cards have few specifications to worry about:
a. Capacity
b. Read speed
c. Write speed
Faster cards cost more. Why? Because they cost more to make, have the latest technology, etc. Like any product.
What will they do for you? They're write and read 'faster' than a slow one. This means once you fill your camera buffer, then the camera buffer will clear and allow you to keep shooting sooner than with a slow card. And when you transfer the files to your PC, they'll transfer faster.
There is probably some validity that the faster more expensive cards will be better made (physically), have more perfected firmware for their internal controllers, and receive better support from the factory like any leading product. I'm sure this part is minimal, but probably present.
Also know, your 350d can only write/read so fast itself. Because it's relatively old.. newer CF cards could easily write/read faster than the camera can write/read.. which means you could "overbuy" for that camera.