Do both: take some yeast from the first ferm'n, stick in a bottle in fridge, settle, decant and add fresh clean water, settle. Prep a starter soln: Y nutrient, DME, dextrose, water, add a thick slurry of the Y from fridge, aerate well, stir. Let it go until it settles. You can use now, or give it another run in similar solution. I like to generate NEW yeast cells.
I've used Wyeast1728 from bottom of a year-old Ale, made successful starter, good ferm'n.
Larger starter, better chance of good ferm'n.
Better way: Buy fresh Y, make large starter, ie, twice what you need. Use half, find how to freeze other half. Thaw and repeat. This way, the Y is ALWAYS new, and you save $$$.