Client Bought Out Company - What To Do About Their Access Control System?

First things first is you need to check what type of readers and credentials they are using between both systems. Companies are using either of HID Prox II, iClass, Smart Cards or some propriety card/fob. Do both of these companies line up in that department? If they do great your life has been made much easier as it gets down to a chose of software. If they don't, then you are going to have factor the cost of replacing readers to head end you chose.  

Next you are going to want to make check the age of the systems to make sure their door controllers are in good shape. Some of Kantech systems are known to be bit older vs. an S2 system. Some door controller might not even be IP.  Do any of these systems integrate with an alarm system or camera system is another question you are going to have to find out. It might come down to which system is bigger is the one you go with because of the cost factor too. Age of the system will be also be a factor. Most companies like keep their access systems as long as they can before they even think of upgrading.  

It totally depends on the organization's security needs.  In most cases though, I would recommend standardizing on S2,  I am a big fan of S2.  

You should ask your customer, "who will manage the system after their merger is complete?"  and "Do you require easy visibility into this new addition's systems"

taking a moment to better understand your customer's business needs and objectives will go along way towards providing them a meaningful solution, and solidify your relationship with them.

If their new facilities will retain their existing infrastructure management teams, you will have an argument for leaving as is. but if that staff is leaving, or if they are aiming to have a more centrally managed system, then S2 should be the way to go.

Just a disclaimer. We don't sell S2 or Kantech so I don't have a bias toward either one. 

I might start by assessing the cards issues, readers, controllers and any integrations like DSC panels working with Kantech access panels as previously mentioned.  Is S2 integrated with their payroll system?  Small details do matter.

Which group has more?  Sometimes the company acquiring has a significantly smaller investment in security, or reverse. 

There are a few key points before a real suggestion can be offered up. 

All things being equal, my vote would be for S2; as you know however, things are not always equal (see everyone else on this post).  My view is forged by 30 plus years in the system consulting and design business.  During that time,  I never had Kantech  call on me, send me an email, invite me to a webinar or bid on my projects. Do not see them much in the trade journals or have had integrators I know approach me on this product.  Why?

The devil is always in the details. You should look at both to find out which will fit present and future needs. Finally, You said the owner uses S2. This alone should put a check in the S2 column (assuming the owner is satisfied with the product).  

Thanks for all the feedback! Interesting to see the support for S2.

But there is there any way to keep both systems as is and manage them as one - like psim for access control?

The first thing I would do is talk to your client and see how they would like to proceed. You may be making plans that are a waste of time. If you have a good relationship with them this should not be an uncomfortable thing to talk about. Let them know that you are looking to help them make the best decisions in their security and you are looking ahead to prevent future issues that could happen with either or both systems. Like card incompatibility, system administrator(s) or server location. I have worked on both and each have good and bad sides. Just my opinion.