What do you need / want your VMS to do? For example, integration with 3rd party systems like Lenel, video analytics, intrusion detection, etc? What type of advanced functions / management features do you desire?
Also, what version of OnSSI is being offered? They have 5 'feature sets' / versions. If it's not the highest end one, what's the cost to upgrade to higher versions?
Just another note on this. I don't see myself tied into any vendor right now. I can just as easily switch to another platform if it meets my needs. I am sure I can give the right incentive to another manufacturer to flip the VMS switch.
It sounds to me like you're in the perfect position to take a little time and have your integrator explore all the potential options. If you don't have an integrator, you could reach out to several in your area and do an RFI. Then, whichever one gives you the best overall design might be a good selection to provide you with a complete design/build quote to get you into whichever system really is the best for your application. Whether thats Lenel or OnSSI or some other VMS that might be even better for you - now would be the time to look into it AND, my advise would be to select the most open architectured system you can. Ask them to recommend a system with heavy emphasis on future proofing. Another thought would be to spend some money on hiring a good security consultant with experience in systems of your size and complexity. Good luck.
If you are not virtualized yet you may want to consider it. I use OnSSI CS across a campus and have virualized all the recorders and base with no problems. Pretty smooth transition. I dont have experience with Lenel though.
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!