Can't speak for the Curve... but I love the Pearl.
Keys are relatively large for being (sort of) QWERTY device,
and the scrollball itself is a wonderful technology and works incredibly well.
Responsive, and IMO it if it were any bigger, I wouldn't be able to blaze through the menus, options, and whatever else I use the trackball for as well, or as quick.
I held off on getting a BB just due to the side trackwheel. When the trackball came, it was a no brainer