@BartS, you don't understand market capitalization. Market cap is the price of the coin (or investment) multiplied by the total number of coins (or shares) available. The price drives the market cap, which is why the price is so important.
And the comment of yours that I've bolded doesn't follow logic. The size of the market cap doesn't drive or influence buying decisions. Market cap is a consequence, not an influence.
When comparing ETH to BTC, ETH has a greater likilihood of growing 10x than BTC does. And LTC has greater liklihood than ETH, and so on. The OP's question was about whether to invest in BTC or ETH.
Given the size of your investment ($1300), I'd consider going all in on Ether.
Bitcoin and Ethereum both have very good long term growth opportunity, the challenge is that Ether is way earlier in its run than Bitcoin. With Ether currently trading at ~$250 you can buy more Ether than you can bitcoin and it's the number of "shares", coins you own that will give you real leverage to gain return on investment from any price growth in the future.
It's feasible that Ether gets up to the $2000s level and beyond (just as Bitcoin has). And with a cost bases of $250 you stand to make a 10x return on investment. However it's less likely that Bitcoin would take the same 10x run over the same time period...do you think Bitcoin could hit $25000 in just 3 years?
Welcome to Bitcoin!
Price is not the most important thing, it is the market cap that matters and now that
the market cap of bitcoin and ETH are closer then the logical conclusion is that wit will be very difficult for Ethereum to grow 10 times its current price, if you wanted to win that amount of money with Ethereum you are one year too late.