If you are really worried about accuracy in your power meter, this is actually the better question. Power meters that only measure the output of a single leg will not be as accurate as power meters that measure power from both legs. The confusing part of this is that the specs might still claim to be accurate to within +/-1%.
What’s happening is that humans are not machines and your power measurement won't necessarily be perfectly even between legs. For me, the numbers show about a 2% strength advantage in my right leg. That means even if a power meter captures the measurement from my left leg only, that number will not accurately represent the total power I am able to put out. Bottom line, a single leg power meter will be repeatable but not accurate when compared with a dual leg power meter. You will need to decide if the lower price is worth less accuracy when deciding between dual leg or single leg measurement options.
For many people, it will make sense to start with a single leg power meter because of price. If you are looking for the best budget power meter, it will be single sided. If it's all you have and use, you would likely never know the difference so that won't be an issue. Also, many of the single leg systems are upgradeable at a later date if you decide you need more accuracy.
As you make this choice, there is one consideration to keep an eye on. When a brand tells you that it measures both right and left leg power, that does not mean it is a true dual leg power meter. It's very obvious to understand that with a pedal you either have a dual leg system or a single leg system. It's less obvious with a crank based system.
If you choose a crank based power meter then often there are strain gauges measuring the combined power from your left and right leg. You could define that as dual leg measurement but nothing is physically measuring how hard each leg is pushing. A system like that will be accurate for the whole number but may not show leg imbalances as well. If knowing exactly what each leg is doing is important to you then choose a system that has separate strain gauges for each leg.