Ross,
The explanation by the running shop should have been that the shoes you bought suit your feet/build/running style and the types of runs you do e.g. on pavement, grass etc.
Presuming this advice is good I would keep the Saucony's, but would NOT wear them until after the FLM, presuming this is the biggy race you are mainly interested in.
If you are not injured by wearing the old shoes and get no problems in running longer distances in them they use these for training & the race, while breaking in the others by wearing them as casual shoes or if you do a small 1-2 mile run.
Good luck,
Rob