Ethereum can be a short-term investment while bitcoin can be a long-term investment so that will depend on you which coin you will buy later.
I wouldn't use the term "investment" for altcoins because they are horrible as investment since they don't really have any real potential for rise and almost all of them are in a bubble.
Altcoins are mainly good for short term trading, specially when they are being pumped and dumped but obviously pump and dump is a very risky thing to get involve in (it is a form of market manipulation and very unpredictable).