I'd always choose Virgin if possible. The longest outage I've had is a couple of hours, but according to thinkbroadband I've not had an outage in 8 months.
Currently on Vivid 300, which is 300 down and 20 up and I don't ever see any deviations from those speeds.
If you're limited to ADSL, I'd be tempted to pay for a business line from Claranet