For short-term trading only you can play with LTC vs BTC.
LTC coin is as good as BTC in use, but it doesn't have BTC awareness. Investment means longer term and only BTC has such value at the time. If you plan to invest and hold the coins say for a year, then I would stick with BTC. whatever the rate (below image), but this is it.
Now the prices of litcoin is declining so investing in Bitcoin would be suggestible (only for long run like businessman )
you can now trade at best exchange rate
*Please invest only that money which you are willing to take risk from. (Do a research before investing)