re 5x5 vs periodization & more:
**FRAT warning**
First, it depends on how you're doing 5x5, as there's a million ways to do it. Some consider the first two sets as warmup/buildup, and then 3x5 with "working" weight. Some do all 5x5 with working weight. Some add weight every set, intending to hit a (near) 5RM on your last set.
So there's that.
It also is gonna depend on your progression - are you trying to add 5lbs a week a la Rippetoe's Starting Strength or?
That's then gonna lead into your training history. SS is great for noobs, and they tend to make a lot of gains...for about 6-8 months. Then they stall because you can't add 5lbs a week forever. Nobody can. That's when you have to alter your programming.
How your entire program looks matters, too.
e.g. - a SS-style 5x5 is gonna look *way* different than say an old school Bill Starr Heavy-Light-Medium 5x5
So there's all that.
As far as it vs "periodization" - too broad of a statement / question, as it'd gonna depend on the type of periodization. Long-term linear periodization is obviously effective, but there are better options for your "regular guy" (unless you wanna put good money on the fact that your "regular guy" is gonna faithfully and consistenly stick with his program for the next year for an entire linear periodization model to play out...which about 99% of guys don't & won't do).
Then there's conjugate periodization & concurrent periodization, either (both) of which can be a better fit for your "regular guy".
All that said, it's not (or at least) shouldn't 5x5 -or- periodization (no matter the style), as 5x5 can very easily be worked *into* and as a part of your periodized scheme.
Ultimately, it depends on your goal, where you are in relation to your goal, training history, your size, what kinda shape you're in, other associated factors (age, recovery, diet, stress levels, etc, etc, & so on).