Directional: Pick the ad that is in the lead at this moment. It couldn’t pass through any peer-reviewed scientific journals, but you’re still basing your decision on recent performance.
Referential: Look for trends in messaging within your high volume ad groups. If it’s the same basic test across both ad groups (i.e. one main benefit against another), you can reasonably infer that the ads will perform similarly. Ensure that whatever significant tests you refer to are similar enough to make sense. You wouldn’t want your successful messaging about shoes to decide how you’ll sell vacation plans. Boots, however, could potentially apply the lessons from shoe ads.
Indeterminate: Continue to wait until your tests get more information, possibly enabling auto-optimization for your ads. Not acting is a defensible decision, but it won’t improve your performance immediately. Low volume ad groups are almost always going to continue being low volume, so the sooner you get comfortable making decisions with limited information on those ad groups, the sooner you could start to see performance improve.