I simulated a million stat arrays with each method. I sorted each array, then took the average. Here's what I got:
with 3d6x12 method: 10.8, 11.5, 12.2, 12.9, 13.9, 15.2
with 4d6 method: 8.5, 10.4, 11.8, 13.0, 14.2, 15.7
So, with the 3d6x12 method, your best stat will be on average 15.2. With the 4d6 method it will be on average 15.7.
Conclusion: with the 4d6 method, your best (and second-best and third-best) stats will be higher, but your remaining stats will be dramatically lower.