I think you should buy both Bitcoin and litecoin as both coins have tremendous amount of potential to grow, Bitcoin being the master of all crypto is without any doubt the most popular crypto available in the market. You can easily expect it's price to hit 100k$ mark within 2-3 years for sure.
Litecoin on the other hand is 4 times faster than Bitcoin and is more easy to use in the real world because of its high transaction speed. You can expect it's price to be around 4k$ within 3-5 years for sure. For higher returns invest in both of them.