I think this is a tough call. But I just did a quick check on Swappa.com and I'm thinking that if I looking to sell in 30 days the S5 would get the nod over the 5S.
However, I think the Note 4 is due out soon. While not a new version of the Galaxy S-series, it will probably have some impact on S5 pricing.
Also keep in mind that the new Galaxy Alpha is just rolling out. So that will have some impact on S5 resale value.
Considering that you don't seem happy with the S5, I think I would return it and get the 5S. You seem like you want to stay in the Apple iCosystem and I don't think the difference between the two will be more than $50 either way.
However, I think the Note 4 is due out soon. While not a new version of the Galaxy S-series, it will probably have some impact on S5 pricing.
Also keep in mind that the new Galaxy Alpha is just rolling out. So that will have some impact on S5 resale value.
Considering that you don't seem happy with the S5, I think I would return it and get the 5S. You seem like you want to stay in the Apple iCosystem and I don't think the difference between the two will be more than $50 either way.