There's probably more experience with Webkit, and given that its already integrated with Qt, it makes sense to go that route. Plus, Symbian and S40 both use Webkit-based browsers, and so this makes for easier porting of WRT and other tech between them.

Hopefully, Nokia will unifiy the engine, controls, and UI across these platforms. This move would make a lot of sense for them.

For Mozilla, it would be a good play as well, as they'd get to play a good bit more with making the browser-as-a-platform, and work out the usability of Weave on OSS and closed systems.