My personal opinion: low mintage is always a factor (how much of a factor is debatable).
2015 ASEs were minted at about 45 Million last year.
2015 Somalis far less. They are minted by the Bavarian State Mint and the last numbers I have are for 2013: only 130,000 BU coins. I don't think the numbers have dramatically increased (my opinion).
I buy both coins (BU as well as MS70 examples). But I like coins that are 1.) beautiful to look at (both meet that criteria); 2.) High production quality (both meet); and 3.) low mintage.
So given a choice (which I personally don't have to make one), I'd go with the Somali.
And there is even a rarer version of the BU Bullion Somali elephant coin: the 'privy seal' versions. Same coin, but the addition of a small privy seal of the lunar year animal. 2015 had only 3,000 of the Ram Privy coin minted. Costs more, but rarity is often a factor if the future.