I think that AtS had the stronger pilot, mainly because it included to very established characters that we all knew and understood. We knew what Angel's story was, his reasons for going to LA, and we knew what happened to Cordelia and why she would think that LA was the only option for her.
The writers knew these characters, they were able to write them in a believable way and the actors (CC and DB) knew their characters after playing them for three years at that point and I think for that reason they had a stronger opener. Plus, it also matters that BtVS' opener was, in fact, a two-parter, whereas AtS was just one.