They are very similar systems, so I would imagine overall they would have similar reliability.
In favour of the rMBP 15", you can say that because the current ones now are a new revision, they hopefully ironed out some small issues that the first ones had.
On the other hand, the rMBP 13" has lower power components (no dedicate GPU, 35w dual core CPU), so you may have less heat related issues in the long term.
Overall, Macbook Pros are very reliable, as laptops go. Best you can do is be careful and look after it. But at the end of the day, as other's have mentioned luck plays a big part - I've had machines I treated with kid gloves give issues, while friends of mine have caused all types of dings without any issues.