$1500 on bitcoin, and the rest diversified on other 5 coins (the ones that are on the top 7 at the moment) that would be the best thing to do, at least, for me.
It's good but if i have $3k i will invest full amount in BTC, yes if i buy BTC in my local market i may get 0.5 BTC and every month at least once the BTC price will vary min $300 to $500 sometimes $1k. This profit is enough for me. I agree if we invested in altcoins we can earn double profit but no guarantee in BTC also no guarantee but we can trust BTC to compare other altcoins.