My dealer carries both lines and I have heard the Electra's and B&W Diamonds many times in the same high end Moon system. It was always the towers used, for Focal it was the 1028's and 1038's and for B&W it was the 804's and 802's. All speakers were the latest models. There was also an Alto Utopia there for a time when the 804 was in the room.
My impressions were I always preferred the diamond tweeter to the beryllium. The 1028's and 1038's were on average all around better speakers than the 804 with the exception being the treble but they had better mids and bass.
The 802's for me always surpassed the 1028's and 1038's across the board. Comparing the Alto to the 804's, well the Alto wins there
I would also give the edge to the Alto over the 802's. For treble the advantage goes to the 802's but the Alto wins hands down in the mids and bass.
This is not the case though with the 800's. The 800's bring everything I liked about the mids and bass in the Alto along with my preference for the diamond tweeter. I sure wish the dealer had the Scala's as I've always been curious how much improvement there is over the Alto.