Both of this coin have their own advantages, both of this coin give you options which you will choose for your investment, If you want to long term investment and don't wanna use that coin for any activity or only hold it, I recommended you to choose Bitcoin, because this coin have good prospect in the future.
For ethereum, you can choose for trading investment, because I have seen this coin can fall and rise in some days even some hours. This really matches to get some profit.