Back in the forum after a long time, so just my two cents.
The Dolby Surround is Discrete L+R matrixed into rear channels through Pro Logic, with no discrete frequencies.
This format is called the Dolby Surround.
AC3 5.1 has 6 discrete channels which means better frequencies for LFE as well. This technology is called Dolby 5.1 Surround. And, obviously AC3 being an advanced technology is much better.