For stamina and consistency I'd recommend playing 16ths, and perhaps playing 4/8 notes per string at gradually increasing tempos.
You can give your left hand a work out at the same time by playing something like:
------------------------------------------------------1-2-3-4-4-3-2-1--------------------------------------
------------------------------------1-2-3-4-4-3-2-1------------------2-3-4-5-5-4-3-2--------------------
------------------1-2-3-4-4-3-2-1-------------------------------------------------------2-3-4-5-5-4-3-2-
1-2-3-4-4-3-2-1---------------------------------------------------------------------------------------------
Etc etc etc. I say practice groups of four as playing triplets tends to come naturally with 3 fingers, but playing groups of 4 evenly can be tricky, especially when you up the tempo. Work on making sure each note is even in volume and attack (for this reason, it's important to start slow and gradually build up the speed).
I tend to use ring middle index then repeat it, so each time I have a different finger playing the first note in the bar. I've played like this for so long it feels natural now but it can take a bit of getting used to, especially when string skipping. Practice makes perfect!