| Both! That's what I am doing, but then I play with guys who like doing both on guitar and they are my two favorite genres. At the risk of offending a few folks, playing blues is easier. Rock is a much wider category and there are many songs that you just have to learn explicitly, where in blues there is more of a formula and variations. To play blues very well is not easy, but to learn enough to be able to provide a bass line to almost any song given the key and description (like "This one is a shuffle in D with a fast four; it starts on the five but you don't come in until the one") is not all that hard.
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