If you are in the U.S. a couple of things come to mind.
First, you have a decent employer if they are making this offer at all. In the U.S. a salaried employee has no set maximum hours and an employer could require you
to work 50 hrs under many circumstances without additional compensation (there are of course limits). But in general your only real recourse is to leave if you don't like it.
Second, The fact the employer is using a carrot and not a stick is also a good sign, e.g. I have seen employers go to employees and tell them to quit their second job or risk their primary one, and then force them to sign a document stating they won't work for other employers without prior approval which will never happen. Some states have protections against this behavior but not all.
In the end, to me the money wouldn't be the biggest factor, it would be what do you want to do? If you are happy doing side work or your second job and that is fulfilling and not just about money this isn't worth it. If you are doing it solely to earn extra income, than this is an option.
If you are interested in their offer, I would not approach them with the argument of 25% more hours deserves 25% more pay. I'd approach them and say you respect their offer and are very happy they consider you valuable, but the second job is already earning you more and you are enjoying it. So you want to point out that essentially their offer is to give you less, without being an ass about it. You want to imply, not demand here, be humble it will go further. If they are really interested in your extra time and they seem to be good people they'll automatically offer to go up some more or give you other options.