Important factors that come to mind:

1) Purity - Is it tested to be verifiably free of contaminants?

2) Dose - What is the CBD/THC ratio? (I.e. 10:1, 1:1, 1:10)

Higher CBD content with lower THC levels tends to be more effective as sleep aid. While some find that balanced or higher THC ratios are more effective for pain relief.

Although THC is often associated with pain relief, it is through a different mechanism (mood and psychological) than painkillers which block pain receptors in brain.

3) Consumption practice - How often is it consumed? Is it being mixed with consumption of other substances? Alcohol consumption should generally be avoided for optimal health, but particularly when CBD or THC are present in the bloodstream, as liver and kidney toxicity will be amplified.