I'd say all things being equal, you're likely to pick up a dodgy tank of momentum than V power. Far fewer outlets of Tesco 99ron, meaning it'll most probably sit there for longer compared to Shell with a higher turn over of customer 'for' V power. Logically if Shell are shifting more V power than momentum, obviously the tanks are being replenished more often.
Don't have stats just solely going on the number of outlet I see here in the Midlands. Places whatever the industry tend to have more outlets, when serving more customers.