If you run a masternode, you're essentially being guaranteed passive income.  If you run it for a year, you *will* earn some dash and you can check that earnings calculator to see roughly how much. 

How much you earn in terms of fiat currency depends of course on the price of dash.  You're right about dash possibly not performing as well as bitcoin (and in my opinion it might not), but bitcoin doesn't earn any income whatsoever.  You could sell off all the dash you earned throughout the year and grow your bitcoin holdings that way.  If you just held bitcoin you *might* get capital gains but no income.  There's a big difference there.