IMHO, they are both very good and they are both very different. Some may prefer the sound of the SCD1 while others will certainly enjoy the XA777ES to a far greater degree.
For those who never heard SACD before and listening to a SCD1 for the first time - it is truly a shocking almost breath-taking experience. The "amount" of sound produced by the SCD1 on SACD is amazing. The detail is amazing. Highs bounce all over the soundstage in a carnival of excitement. I believe as Czbbcl stated above - this was indeed created as a statement piece and designed by Sony to grab the attention of the world and enlighten them to what Sony hoped was the future....SACD.
Many people were captivated by the SCD1 (and still are) but I found over extended listening, it wasn't fun, relaxing or enjoyable. The unit has a "matter of fact" quality to it and subjects a listener to the music more than drawing one in emotionally. Sitting in a room with the SCD1 - and its detail - was similar to sitting in a room with ants crawling
over my body... it was a sort of organized rhythmic chaos made up of unanticipated notes & sounds which initially sounded spectacularly interesting but ultimately grew fatiguing.
I feel the XA777ES is an attempt by Sony to take a step back and prove that SACD is not an "in your face" format designed for detail freaks or those who love to hear that which has never before been heard on a recording. While the XA777ES is certainly detailed - it does it with a certain ease (and sonic signature) never before available from Sony. Additionally the XA777ES seems to be have an alive-ness, is less sterile, and a certain degree of warmth that is non-existant in most digital gear in general - especially under $5,000.
The SCD1 takes a looooong time to load a CD. Some like that "feature" (others don't)... but it gave me plenty of time to get from the rack to the seat before the CD began to play. The XA777ES loads much faster.
To me, multi-channel is annoying and I cannot understand what all the "hype" is regarding the format. But.. if you must have it, the XA777ES is the unit to own.
The front loading XA777ES is very convenient but certainly not as cool looking as the previous 777 or the SCD1.