>Wait till Vritra
Wait, wait, wait. What? They are both as bad as the other. Rahab should be preferred if you really want to make a choice between the two be it sub or main, as she has better stats (unless you want to focus on having more HP for whatever reason). That +30% burst you're talking about only adds to the burst streak, and not to the individual bursts, so that little use of it would only be seen at burst time.
40/20 eidos & elemental eidos are to be preferred in general depending on the advancement of your grid.