IMHO I would go for the OEM (factory fit) type kit, but get the non-rsap version (like the 729E kit), this means no clutter on the dash and fully integrated with the steering wheel/dash cluster and touch screen interface on the RNS or RCD510

These can be had for around £250 and the install takes about an hour.
I have retrofitted one of these to my Mk6 and it works great with both my Blackberry 8310 and iPhone 3GS (neither of which would work with the factory premium bluetooth).