Why do not you choose something else. I am sure that the third coin will come to the market, which will take up the speed very quickly and take the lead. It will not be bitcoin or etherium, something else. Therefore, it is impossible to say unequivocally who will be the main one. It is necessary to observe the market and watch trends, apparently something should change and I hope that for the better.
I support that the cryptocurrency market does not stand still. It is necessary to be constantly in dynamics and observe what is happening. There are new promising coins, so you can not bet only on Ethereum and bitcoin.
Just a simple rule, an investor should diversify the portfolio, hedging risks etc. There are tons of other coins form EOS to DASH.