P60 drop-in voltage choices?

Hi all,
Sorry if this has been asked before. I searched a little and didn't find and answer.
So, P60 drop-ins seem to be available in 3-6v or 3.7v only. Obviously former is meant to work with one 18620 or two 123's and the latter with a 18620 only.
If I use only 18620, is there an advantage (like efficiency) in choosing the 3.7v drop-in?
The specific modules I'm looking at are XML single mode units. DX has a 2.8-4.2v unit and solarforce has a 3.7v unit; are these the same thing?
Any other recommendations for a budget minded single-mode drop-in?
Thanks,
Walter

Never mind... found my answer!
http://www.candlepowerforums.com/vb...-ins-wide-voltage-as-bright-as-narrow-voltage
To paraphrase mdocod and others: 'most (wide voltage modules) don't reach full output until somewhere around or above 5V input'.
So, if I'm going to use a single cell, the narrow range module is better.