There's no clear answer to that. I think you will do fine if you buy Bitcoins for a long term investment. However, they are so expensive right now that there may be altcoins that can give you a better rate of return. The problem is that there are so many altcoins to choose from that you can't really determine which ones will do the best.
I choose to diversify by holding Bitcoins and a few of the more well known altcoins. I try to stay away from new altcoins because they often will be worthless in a few years.