QML will provide more support for customising widgets in a simple manner, Widgets gives you a set of tools ready to go that gives you platform styled controls in a very simple way. Both have their upsides, both have their downsides, I'd personally recommend getting started with Widgets first as there's a lot more historical documentation for them, so if you need to research something you'll have a bigger base to work with.
From your comment above, there's been little development on Widgets as the Qt development team feel that they don't need any additional work at this stage. They're stable, solid and reliable, there's not much that needs to be done to be improved, and when people need custom widgets they end up creating them themselves.