Why compare two incomparable currencies? Yes, of course bitcoin is the first currency and Ethereum is just an altcoin, but for investment I would choose Ethereum.
They are being compared on the basis of price in the market. Both were initially thought of as potential opponents in the market and ETH was expected to give Bitcoin a hard time in 2020. Besides, both of these coins have been ruling the market claiming the top market caps.
The profitability in Bitcoin was considered to be high than ETH but the rate at which ETH was advancing was viewed as faster than Bitcoin. This is why these two are often compared. And yes investment in both these coins can be profitable than in any other coin.