Using the hardware button is a little slower. To use it, I need to:

1. move my thumb away from the screen (just a little, but why bother if it's not needed?)
2. push the button
3. wait for the menu to draw
4. move my thumb back to the screen (again, just a small, but unnecessary movement)
5. press the menu item for the target app

This is versus one on-screen tap for the majority of my task switching in non-full screen. Worst case, I have to tap the screen twice and suffer (Oh, the pain!) a menu draw. And if I do have to open a menu to switch, I can often use the app-specific switch menu and thereby narrow the number of choices I see and from which I need to select.

I'm not saying it's a big difference, and I don't expect I'm a typical user. I'm just saying full screen app switching makes Maemo feel uncomfortably slow and clunky to me.