The Macbook trackpad has by far the best horizontal scrolling of any pointing device. I could not live without it. (If anyone knows a better solution please tell me. I'm always looking for new options.)

It's great for spreadsheets. I even use the Macbook trackpad for 3D CAD. I used to use a spaceball and mouse, but the trackpad plus modifier keys is more convenient because it keeps the hands to the keyboard.

In the event that you'd prefer a mouse for standard pointing, I do think the Magic Mouse is equally good at horizontal scrolling. One could quibble that the Magic Mouse offers less room on the surface for horizontal movement, but in practice I don't think this makes much difference.

Best of both worlds—

I know many people that use a mouse and Magic Trackpad and fluidly switch between them depending on the task. I did exactly that for about a year—before eventually ditching the mouse altogether.