I've been looking at mining litecoin, but it seems to me that the exchange rate between BTC and LTC depends on relative difficulty so that mining litecoin with my AMD card will give me the same dollar value whether I mine BTC or LTC. If the exchange rate continues to behave the same, then once ASIC miners raise the BTC difficulty a lot, LTC value will drop significantly, so it would make more sense to keep BTC, and therefore to simply mine BTC.

(I understand it's more nuanced than this, and I can see how short term and long term strategies may differ, but I'll let you correct me if you think this is wrong.)