As a dealer for both ONSSI and Lenel, I have a couple clients facing the same situation you are, however I really tried NOT to sell the Skypoint version as I knew it was just a stop gap product until Prism was developed and released. I didnt want to risk my clients being stuck in a non-supported product, and Lenel was not very good at supporting the Skypoint line in my opinion. I have yet to deploy Prism but some of my counterparts in other states have and it has been somewhat painful from what I am told. There are 9 pages of known issues in the current release, that scares me!
Right now I am suggesting to my clients that are in your situation to go down the path of ONSSI as we consider it to be a stable reliable platform that perfoms as expected. I am sure that the same will be said for Prism in a year or so, but right now it seems pretty green.
One other thing is that ES is pretty heavy as far as prcing goes and I have only a couple clients that utilize that level of software. I am sure you have done your homework and require the features of ES over CS, but with OnSSI Mix and Match you can have ES as a base and use CS, IS or even PS licenses (with some limitations) to save money on licensing fees in the future. Just food for thought!