It depends on what hardware you have available to you and what other limiting factors there are, hardware wise. Choosing what to mine is about efficient use of electricity, and miners make these micro economic decisions every day - but within the confines of what hardware they have. A SHA 256 ASIC is only ever going to mine SHA 256 coins, a Scrypt ASIC is only ever going to mine Scrypt coins. GPUs on the other hand, can mine a bunch of different things with just a change of software.