It's funny that in the beginning of the thread people are saying that OP is too late to buy ETH because "the market is dumping"
It was only a few weeks ago and you can see how quickly things can change.
The cheapest way to exchange BTC to ETH is a coin swap feature available on some exchanges like coinbase. It allows you to turn one balance into another without fees.
Of course I wouldn't recommend going from BTC to ETH now because I feel like BTC is only waiting to explode again and ETH gained a lot in the last 2 weeks, so it could correct when Bitcoin starts to move above 40 thousand.
Right now there is not a huge difference at least when it comes to profits when you invest in ethereum or bitcoin but as you say while this is true now I think that during the next months we are going to see bitcoin going up even faster than ethereum as more retail investors decide to invest in bitcoin out of FOMO, right now the price is going up and I think we can expect to see a few new all time highs as the chance for more and bigger profits will be taken advantage by traders once again.