I think it depends on what we have. Some have ETH and some have BTC. So it depends on what you have. Personally, I use these both. because I have ETH and BTC. So I buy altcoins using these both coins. But, most of the times, I using BTC for buying tokens. If you need to make an investment in the ICOs, Use the ETH for that. ETH is much better to it. However, these two are great currencies for use.