Tim,
Definitely fix the old one. My father had a circa 1970 LawnBoy that he used about 20 years without doing a lick of maintenance, not even draining to gas tank in the winter. Based upon his experience, I bought one in 95, used it for light duty. There was no comparison, the ones you find in Lowes are no better than any other push mower. The carburator on mine was made of plastic and it didn't last long with similar neglect.
I guess I also need to qualify my above statments, my father paid around $250 in 1970, and I paid about $200 in 1995. So today, my father's mower would probably cost close to $1000.