Both are good as an long term investments but altcoin market is too volatile and if you are a day trader or believe in short term profits then you can invest in ethereum and take advantage from each pump and dump which will give you timely profits and will also grow your bitcoins in short term.
I really prefer to have both not just because because of investments but also for other functions. It is given that both coins are good as investments but for me these two acts as primary medium for trading. Ethereum acts as gas for ethereum based transactions and for using MEW so it will be a pain in the wallet if a trader would continue buying it from gas site like Shapeshift which has a big fee. Bitcoin has a bigger role than Eth but when it comes to trading, they both act as main tool for buying other coins.
As a trader, you will not have to choose between these two good coins because you will smartly invest on both for trading purposes.