No, HDMI is technically more capable. This doesn't mean there's anything wrong with optical, though, and depending on the source device optical may be delivering the maximum potential anyway.
Allow me to explain...
HDMI is capable of transporting any stream that optical can - and it will be exactly the same. Whatever data travelled over the optical cable will travel over the HDMI cable and be exactly the same when it gets to the amp. The place HDMI can make a positive difference is with Bluray (and HDDVD, if you had one of those) and HD movie files on a HTPC. This is because HDMI is the only standard capable of transporting the new HD audio formats. Optical can't cope with them.
The only time optical will actually be better than HDMI is if the source device somehow cripples it's HDMI output. This is the case with Sky HD satellite boxes in the UK. They are capable of transmitting a full Dolby Digital 5.1 sound track (which you usually get on movie channels) via their optical output, but not via HDMI. The HDMI output is limited to stereo. This, however, is not a fault with HDMI itself, but with Sky's boxes. Hence, you'll see many people connecting both the HDMI and Optical cables from their Sky box to their amp, in order to get the best picture (HDMI) and best sound (optical).
But that's really beside the point and a bit offtopic here (sorry!). Basically, when it comes to HTPCs, HDMI is the way to go! It can carry everything optical could, plus the new HD codecs, plus unencoded 7.1 LPCM.