Maybe I will alternate, just for variety.
Feed what works and what is convenient for you, but try to keep it consistent, otherwise you might just end up with a picky eater.

I would also recommend you to stick with rats or mice, gerbils are more aggressive and have longer teeth which make them potentially more dangerous than mice or rats.

Also if your BP imprint on gerbils this could also be an issue if you had to re-home him in the future (most people will not feed gerbils or do not have access to gerbils)

While rats nutritional value is different than mice nutritional value there is no study to this date on BP nutritional requirements.

Gram per gram rats or mice it's all the same and one is not better than the other.