Personally I would go for a 100% BTC focused portfolio IF I had to choose between the two, but I believe that altcoins have higher growth potential than BTC just because we haven't seen any serious institutional funds pouring into them.
As for bitcoin's movement with these past few months, it is kinda safe to buy now, like a 50/50 situation to win or not with minor spikes. 100% funds on bitcoin isn't that fun at all, you don't even have a chance to get up if you fall the only option for you after that is to wait long. There must be 2-5 crypto after bitcoin on your wallet, I must recommend the trendy ones or those that has a significant growth with past few months. We've seen what bitcoin could reach, we know what it is capable of but it is not always on that phase.
Who knows what heights can them achieve if that would happen?
There are already alts that slowly grinds to top like Verge, You can see its trend on CMC after a partnership with pornhub. Maybe Verge is the next big thing, Who knows?
Diversify.Diversify.Diversify