The best decision on investing your 100 dollars is to just invest it on both Ethereum and Bitcoin, you can do 50-50 when you invest so it wont be much of an Impact when a bubble pops on one of the two which is not that impossible to happen coz "history repeats itself" imo. Both are big projects anyways so it will surely give you a good profit in no time specially right now that its volatility is fast and huge.

Just dont forget to buy when there's a correction, do not buy when its hitting a pump, its pump always has a correction to follow which is a good timing to buy and fill your bags. As you can see BTC price recently goes down to 28k and pump to its new ATH and currently at 33k..
While ETH price also pump to its new ATH from its recent dip to $1200 and currently at $1400.

If you are going to compare both price, you'll see how volatile bitcoin really is compared to Ethereum but both are good investments specially for swing trades these days.