Nucs are small hives with laying queens, brood and workers. They are hives ready to grow as soon as you get them. Package bees need time to draw out comb, have the queen lay and have the hive settle in. Both need to be fed and monitored for early queen failure and mite problems. If the prices for a nuc and a package is close i would recommend the nuc just because it will be more developed as a hive. Most nucs come from local breeders which could also be an advantage.
I've had success and losses with both, if you buy nucs check with the state's agricultural department to see if the supplier has had problems with foul brood or other issues. I got burned a couple years ago with some nucs that came from a local commercial beekeeper, they were loaded with Varoa and Tracial mites, lost them all within 2 months and he would not replace them. Found out later from the state inspector the yard had problems.
All in all I would go with nucs and treat them for mites but buyer be ware