Yo, convert my old MTB or keep riding my schwinn

So I have to commute to work by bike most days now which has turned out to be very fun but im not a bike guy. Ive had a m500 cannondale sitting around I just dropped off at my lbs (local bike shop??) to get the wheels trued and breaks worked on and plan on switching the tires for something cityish. I have been riding a 1985 schwinn super le tour in the mean time but it is tall and kind of slow af (i am getting absolutely passed up by divy and lyft bikes). would I be better of converting my mtb or sticking with the schwinn? kind of a ramble, thx.