I honestly don't think that the amount of the investment should affect what you invest in.
Sure, if you invest in an altcoin that's early on, you might see higher percentage gains, but you should really do the research and just invest in what interests you.
If you like btc, buy $50 of it; if you like eth, buy $50 of it.
That being said, XRP is really cheap right now and you could get a decent amount for $50. Some don't like how centralized it is, but I do think it will have pretty impressive growth in the future.
I think the amount should affect the investment, depending on the overall situation.
If you have very little to invest and wouldn't be hurt too much by losing it, it's more sensible to invest in something a little riskier with much higher upside potential than to go for a slow investment that will hardly throw off any return in absolute numbers.
Investing $50 in Bitcoin will at best yield $50 to 100 by next year. Investing the same amount in something like SONM could yield $200 to $600 in the same time frame.
Of course, the larger the investment the more important preservation of capital becomes and thus the stakes in safer assets should increase. Depending on the risk appetite people should also consider diversifying into stocks and savings accounts once they reach capital in the range of $500k to $1000k and upwards. Personally I wouldn't waste any time on savings accounts since they have no potential of growing money, but some people seem to like that, which is fair enough.
Either way, I believe the amount invested should definitely be a point for consideration when it comes to choosing investments.