I've used cPanel in the past and I'm currently using Plesk and I'm sticking with Plesk. The primary reason why is because I simply do not like the way how cPanel handles user accounts and lack of control of DNS priorities when it comes to MX and SRV records unless you're logged in with the actual WHS and even then I don't like the interface.
But scsa20, what do you mean?
In normal cPanel fashion, the usernames are always in a form of an email which can cause issues with some FTP softwares not designed to accept usernames with the @ symbol in it. With plesk you don't have that, can create a plain normal username or create one in a form of an email, you just have more control it.
Also, when setting up a SRV or MX records, you set the priorities for each server of the record, highest priority is, of course, 0 with the lowest being 50. You cannot set the same priority using cPanel unless you log into WHS and set it form there. With Plesk you can set same priorities right out of the gate, doesn't matter if you're signed in as the server admin, as the reseller, or just the normal user, can set the DNS your way.
Which brings up another point, there's 2 completely different interface with 2 completely different ways to enter cPanel and WHS so need to remember which port to use to connect to cPanel and which one to enter WHS while with Plesk there's a port for entering the panel without encryption and a port for entering it with encryption and the interface changes based off of which log you use. Log in as the server admin? See all the tools and options that's available as the server admin. Logged in as a reseller? See all the tools and options available for the reseller. You just have more control (in my opinion) then what cPanel has to offer.
Also, as most others had said so far, it's available for much wider range of Operating systems. I've ran Plesk on a Linux server (CentOS 5) and currently running it on a Windows Server (so you'll see more lurking more in the Plesk for Windows side more then anything).
What about stability? Bugs? How often Plesk releases new features ?
Plesk can be configured in 1 of 3 ways for the updates. Early Adopters, General (stable) Releases (this is the one that's set by default), and Late Adopters Releases. If you must have the latest releases and don't care about minor (or even major bugs) then you can change over to early adopters (not advisable for a live server, for a test server sure), otherwise stay with the general for the best of both worlds, you'll get all the latest features in a timely matter with very min. bugs (if any) popping up. If you are more concern about being bug free then you can choose to go with late adopters but the new features will come out many months later.
Honest, I'm running the early adopters builds on my live server (but I am only hosting myself and a few of my friend's sites) and it's been rock solid. Only ran into 1 issue which I fixed by remoting in my server and adding a new environment variable and that resolved my issue. Like what
@UFHH01 said, the knowledge base is massive, even though the knowledge base I found was related to a different issue, it was still related and still resolved my issue. Plus can always ask here and there's plenty of people that's will to provide support without you having to spend a dime and if it turns out to be a bug that isn't listed, you have
@IgorG that will report it directly to the development team.
I haven't had a chance to use their actual direct support but even then I find it very reasonable cause it's based off of the license. If your license is through your host provider then chances are you'll have to pay for the support if you want support directly from plesk but even then if it turns out that the issue is related to a bug they credit you back so they don't just nickle and dime you (and in all honestly, about 97% of the time it is an issue on the server side or user error and not the issue with the control panel itself just by looking through forum posts).
Anyways, hope that answers some of your questions of why I choose Plesk over cPanel.