I simulated a million stat arrays with each method.  I sorted each array, then took the average.  Here's what I got:

    with 3d6×12 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 3d6×12 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.