Hillary Clinton might not please Wall Street by picking Elizabeth Warren, but Paul opting for Ted Cruz could very well turn off undecided/moderate voters. Cruz being on the Republican ticket might make it comparable to Barry Goldwater in its image.
Hillary Clinton/Elizabeth Warren - 405 electoral votesRand Paul/Ted Cruz - 136 electoral votesIf Paul manages to define Warren as out of touch and make Cruz be perceived as rallying the base instead of being hostile, then he might be able to eke out a small victory. It is doubtful that Paul/Cruz could really expand the map, however.
Rand Paul/Ted Cruz - 275 electoral votesHillary Clinton/Elizabeth Warren - 263 electoral votes