My personal guess is that Litecoin has peaked; it offers no real improvement over Bitcoin, unlike several other altcoins. Peercoin will most likely overtake Litecoin, but it will take some time. Dogecoin is a wildcard. Note that long-term investing is very different from short-term trading. In an extremely volatile market like cryptocurrencies, I would recommend short-term trading, since it is quite difficult to correctly predict the future.

Mining scrypt-based coins is probably more profitable than mining SHA-256-based coins like Bitcoin or mining CPU-only coins like Primecoin. Mining is designed in such a way that early adopters are rewarded, so watch out for new coins with potential!