I had the same choice when we moved our office into a converted garage next to the house (thanks for your advice on phones by the way). I have Virgin Media in the house but went with BT for the office, mainly for redundancy but also to separate the office and home use as I like to feel in control of the billing and usage etc for work. I'm not sure that last one is a good reason as it costs me more to do that! But it feels more structured and should (?) have a better support response.
I have a long network cable and second wireless point so I can switch if needed i.e. extend the home wireless (and in fact worked like that as it took BT an eon to install the phone and then the internet lines). However I'm pleased now with that redundancy and knowing I can download stuff or use a couple of VPN connections without harming the download speeds for the house.
For real emergencies I have a virgin mobile dongle - slow but I've only had to use it a couple of times (once in a traffic jam to carry on working!).
Have been happy with Virgin although the speeds went sticky when they did their upgrade a while ago - but went back to normal after a small while.