Surge protectors are classified as Type I, Type III and Type III. Type III is 'point of use', for example the type found in some power strips or plug-in devices. Type III will provide some protection to more sensitive electronics in a minor surge event, but otherwise affords the least protection of the three. Type II usually offers a lot more protection but needs to be installed in an electrical panel, so therefore requires an electrician (or a very competent DIYer). (For those types, I recommend the type that plugs into your panel like a circuit breaker, if your panel has the space, all else such as ratings being equal.) Type I can go on the utility side of your home's main circuit breaker and is therefore beyond the scope of what anyone should be advising on this forum, although a Type I product can also be installed in the same place as a Type II.
I frequently install Type II devices as part of my work; I've never really noticed any joule ratings for those, and I'm inclined to agree with those commentators online who say a joule rating is meaningless marketing for Type III devices. I think the more meaningful rating is the kilo-amps (kA, yes, thousands of amps) that the device can carry and still function. For Type III devices I would look for devices that provide such a rating and consider any joule rating only secondarily, since I'm not even sure joule ratings will be apples to apples.
The effectiveness of any SPD goes up with the kA (and/or the response time rating, but they probably won't tell you that) and goes down with the severity of the surge event. Many devices that would do some good given a somewhat distant lightning strike, for example, may not do any good if lightning strikes right next to your house. Similar with surges from human made sources (i.e. the power company). So there's really no way to tell how much of a joule rating or kA rating you 'need', it depends entirely on how unlucky you are with any surges. Expect higher rated devices to cost more. You will generally get what you pay for, but may never need what you pay for, just like with insurance policies.
There is no harm, except perhaps wasted $$, in piling up multiple SPDs; each will provide more protection. Say, a Type II in the electrical panel and a Type III that you plug the layout into.
