The regular Hyper 212 has a copper mounting plate that connects to the heat pipes. It makes the unit easier to manufacture, but it also reduces the efficiency a little and makes it a little taller and heavier. The Hyper 212 Plus has the heat pipes come directly in contact with the CPU, so it can do a better job.
The Hyper 212 is also made up of two separate heat sink towers to allow for easier air flow, but it also reduces the cooling capability of the unit. The Hyper 212 Plus is a single heat sink tower, so it has more area to remove heat.
Normally you would always want to get the Hyper 212 Plus, unless you are worried about the weight of the heat sink.
You can see for yourself by looking at the pictures of the units here...