If you have absolutely no interest in other areas of radiography, then go for it. Taking the radiography route will give you options in case you change your mind; jumping straight into sonography means a sonographer you will be.
Unless things change, the NHS funding consideration is irrelevant now. If you're applying for Sept 2017 intake you'll have to pay the course fees for radiography. Starting salary for a sonographer is significantly higher than a basic grade radiographer and opportunities to earn more come thick and fast as well.
I actually wasn't aware there was a direct access BSc route into sonography. It's quite a lot of responsibility when you consider you'll be reporting your own scans. I would imagine it would be an easier transition to go in from a radiography background, but if it means you're equally qualified at the end of it, then I guess it doesn't really matter.