bitcoin is the only cryptocurrency that has a considerable amount of real demand for it. that means the price is real for the most part and it can be seen from the resistance it has and the demand to buy it up even in this uncertain times that the media is filled with lots of FUD.
to buy bitcoin, any time is a good time. bitcoin is the winner of long terms which means its price will rise in long term even if it has small drops in short term.
ethereum is very different. it is "noisy", they advertise it a lot but it is in a bubble because there has never been any real demand for it. the rise of its price is purely because of 2 things:
the ICO scheme that forced people into buying it
the hype coming from massive advertisement and lies that people FOMO bought it.
investing in it at this point is highly risky in my opinion.