Need to know the following to make an informed decision...
What is the vialibility date on the vials?
What is the OG of the brew?
How many gallons are you shooting for in the fermenter?
How do you agitate your starters? (e.g., stir plate or manually?)
IMO, mrmalty leaves a lot to be desired. I know a lot of HBT'ers use it, but I found it to be pretty vague, which is why I use and recommend yeastcalc
. No comparison between the two, as far as I'm concerned.
Edit: According to yeastcalc, a single vial with 3 oz. dme/.8 liters water will give you 187 billion cells, assuming a viability date of 5/1 (83%). The same starter size using two vials (same viability date) will give you 272 billion cells. So, based on this, it really makes no difference (you only gain ~2 billion cells) if you use both vials for the starter, as opposed using one vial for the starter and then direct pitching the 2nd vial into your fermenter along with the starter.