In my opinion, all things equal, older collectibles are more desirable than newer ones. There are definitely exceptions to this but I am speaking in general, especially in regards to the "first of a kind"/classic ones such as Casascius.
Any chain split would be a natural separation/distinction because as you said, ones funded prior will contain both sides of the split. They would absolutely be worth more, as long as the price of BTC + Price of New Coin > (is greater than) the current price of BTC. Collector interest and associated premium is the cherry on top. Can't imagine coins would lose value solely because they have "more than their face" or anything like that.
Are there any ETH coins made before ETC? Would be interested to see how those faired before/after the event.
Dont know of any funded eth coins that came out, maybe that monster coin that elianite is selling off but other than that i think just diy coins