Archdemon
Some random dude on the Internet
I think it is pretty important to be well rounded, and we could very well use an increase in HP, as well as melee damage. Therefore, I think the most logical choice would be to choose the Strength upgrade.
(I'm also wondering if this is the last we see of Ethan. It would be a waste to discard him after the time spent introducing him. Perhaps he could volunteer to go with us for a variety of reasons. Of course, this is just a suggestion; feel free to do as you please with him, especially if you had other plans in mind.)
I agree, while our best stat is intelligence, having more of the other stats is important especially when we get in situations when our magic won't be useful.