"Collect what you like," does not apply in this situation, so don't say that. I can purchase a 1908-S Indian Head Cent in VF or an Edward I penny mule, class 1D/2A, for the same price. Both would add to my collection, both would bring equal enjoyment. Which would you get first? Same price, same rarity, you collect both, which do you go after? Dealers pictures, not mine.
I always go for the one that has more going for it. Readily available key date vs. a very old, silver piece that is also a mule and is not nearly as readily available. I may be biased being a world coins guy though
Again, equal rarity. Maybe you haven't seen this mule, but it is a very common mule, as far as the series goes. Not the first 1D/2A mule I've seen, not the first 1908-S I've seen. The 1D/2A mule is noted by the Fox brothers and JJ North. It's not rare, uncommon at best. 1908-S is uncommon at best. Search and you can find either.
My point is not about rarity rating, but about availability. Go to eBay and type in 1908 S indian cent, over 700 results. Type in Edward I penny mule, you get 2.
eBay is not the tell-all of how available a coin is. I'm willing to bet a 1945 Lincoln Cent appears less often on eBay than a 1909-S VDB. By your logic, the 1945 Cent is less available than the 1909-S VDB.
By my logic, 700+ to two means something. But hey, whatever you want to do man Edit: also note that people don't list things like 1945 cents nearly as much because it's not worth it unless they are high grade. That is not an issue with either of the two coins in question.
If I can only have one I buy the mule, I think. The price on the IHC does not begin climbing steeply until mid-AU, and it's easily available due to its' demand. A few more bucks gets a much nicer one if you're patient.
You can walk into any coin store either brick and mortar or virtual and purchase a 1908 S Indian Head Cent. How many mules of the Edward I are out there, and how long would you have to look for another one if you don't purchase this one?