Depends on the context and specifically the relation you have with your newsletter subscriber and they have with you.
If you want it to feel personal a plain text email (or a HTML version that looks like it*) is probably better.
On the other hand, if e.g. you are a bakery and doing a sale on your newest cakes you want to include photos, etc in which case a rich HTML email would be a better fit.
Reason you might want to go with an HTML version even if you make it look like plain text is because it gives you more flexibility in terms of adding tracking links while still making them look like regular links, etc.
Depends on the context and specifically the relation you have with your newsletter subscriber and they have with you.
If you want it to feel personal a plain text email (or a HTML version that looks like it*) is probably better.
On the other hand, if e.g. you are a bakery and doing a sale on your newest cakes you want to include photos, etc in which case a rich HTML email would be a better fit.