Sky seem to limit at 40MB, BT are offering 63MB so they seem the favourite at the moment.
Sky have their "Sky Fibre Unlimited Pro" service which is upto 80 Megabit and would give the same estimate as BT. Its £30/month rather than the £20 for the upto 40 service.
With Sky however you are under terms to use their router, and they make it harder to swap but many people have done so. Draytek and Billion have beta firmwares to work as replacements for the Sky one. (Plugged into the Openreach modem).
I tried to get Sky Pro but they got a rejection from Openreach, so my fallback was Infinity. Given you have to take Sky phone with Sky's broadband products (or BT phone with BT Infinity) the total price is very similar.
James - BT Infinity - 19th Sept 2012 - Sync: 52/12 - Test: 50/10 - Est: 44.6/6.5 Mbps
13 years of broadband - ntl:(512k/1M)/BTbusiness(2M)/Metronet(2M)/Bulldog(8M/16M)/BE(16M)/BT FTTC(50M)