Both ships are great.
Princess is top of the line in service, food , entertainment.
Both itineraries are good.
Depending on where you live and how you want to get to the Embarkation port is where I would look into. Princess goes out of Brooklyn and Explorer in NJ.
There are many reviews on many sites, start to look for them. Take them with a grain of salt as is said. Don't read to much into a review. Look for what you want out of the cruise.
Personally, I would choose Princess as my first cruise.