I've been to both and enjoyed both but for different reasons.
I think I like Neoshoegal's description best though. Fun vs Magical.
MNSSHP is fun in atmosphere - costumes, candy, both guests and castmembers seem to be more open and friendly (I think the (sometimes amazing) costumes break down some of the barriers between people). MVMCP is more 'magical', snow on Main St, hot chocolate and cookies (usually appreciated all the more because it can get brrr at that time of year), the castle looking AMAZING with little white twinkly lights to make it look snow covered....
I hope I'll never be forced to choose - it's like having to choose a favourite child!