Havent tried a VMG3925 myself, but there is a thread about it
here.
Although both seem ok, IMHO the BCM 63168 is the better chipset than the BCM 63381 which is in the VMG1312-B10D
Please note that as mentioned by J0hn, the letter at the end of the VMG1312-B10 makes a difference as to which chipset it contains.
I and a few others have a VMG1312-B10
D - It is very good value for money and as such I can't fault it.
However I do personally prefer my VMG8324-B10
A which has the better BCM 63168 chipset, the '
A's also tend to have the internal noise filter, which is why they are more expensive.