Up to you.
The overall trend we've been seeing right now is that they're both going up. Although, just like other currencies are tied to the US dollar, litecoin is, at least somewhat, tied to Bitcoin. Though, with Litecoin being added to Mt.Gox (eventually) and ASICs shipping out driving bitcoin GPU miners to litecoin (eventually) litecoin is likely to go up in value against the bitcoin and is thus a good investment. Besides, bitcoin going up won't lower litecoin's value against it too badly, so you'd be likely to profit off litecoin if bitcoin went up too, although you'd profit from bitcoin rising more holding bitcoin directly incase people try to trade litecoin at the value that litecoin was in USD before the rise.