Here's how they're the same:
They use the same locks (so even if you mix and match, you can still have them keyed alike), the same method of mounting, and the same sensors. They can both be mounted with either the standard loop tether or the AnchorPoint tether. They're the same height (1.3") and the same width (2").
Here's how they're different:
The Sonic Shock Plasma is longer by 1.7 inches. This gives it a 50% larger bonding area to cling to your equipment. And it has twin sirens, making it not just louder, but more disruptive. That said, the '5' is still noxiously loud.
So how do you decide?
There are no fixed rules. For some its based on available mounting space. For others, its an economic decision. At current prices, we find most customers choose the Sonic Shock 5 for 37" and smaller televisions, and the Sonic Shock Plasma for televisions larger than 42 inches.
Other factors might include:
If you've already had a theft, have a more isolated setting or a situation where thieves have time to work, you might consider the Sonic Shock Plasma for even a smaller television. If you've never had a theft, or you're device is in a well traveled area, the Sonic Shock 5 will be adequate, even for a more expensive set.