We have our clients spread out between a Plesk VPS, a Dedicated CPanel and EngineHosting. We don’t have the latest version of Plesk because the upgrade will be more difficult than we would prefer to deal with just yet (new IP’s. . . don’t ask).
That said, if you can’t use EngineHosting, then I would lean towards this latest version of CPanel with WHM. Both Plesk and CPanel are a mixed bag. I like the way Plesk handles email aliases much, much better than CPanel, but CPanel does a better job of “parked/aliased domains” for my tastes.
If you like to sleep nights and party weekends, I highly recommend you look at TouchSupport.com. Yes, its a little more money each month, but you are worth it.
Of course, the ultimate is EngineHosting, if and when its a good match for your client’s needs.