There is no such thing a 100% chance of success, everyone has a chance of failure. I would not put ESA more than a few % more reliable. Math says your satellite would need to be really expensive for the price difference to be worth it.
That "little" difference has huge huge economic repercussions, cost of insurance, ease of financing, and many other things. Not to mention that a satellite going boom can kill an operator if it doesn't have a backup. This isn't an insurance case where you get your money and try again the week after, building a satellite is a huge undertaking it takes years to build, years to negotiate the contracts for and you have to do it from scratch all over again if it goes boom.
Launch 1 costs $600m: total cost, $800m.
Launch 2 costs $200m: you can build 2 spare satellites for the same cost (assuming you don't pay for the launches that go boom; and even if you do, that still gives you one spare satellite).
Space X has had 29 launches, 2 failures and 1 partial failure (F9-004) so a success rate of 89.6%.
Ariane on the other hand hasn't had a single failure or partial failure since 2002 so if you compare since Space X started its services it has a success rate of 100%. If you take the whole history of Ariane it has indeed a success rate of 95.2% or 95.4% for the Ariane 5 version but the fact that Ariane has flown 72 consecutive missions without failure (2002 - Present) compared to 14 for Space X (2012 - 2015) says a lot about their reliability.
