For advantages, we don't know exactly what yet. Right now hotel guests have access to parks 1-hour earlier, with the actual park alternating (i.e. Tuesday, Thursday, Saturday it's at
Disneyland and Monday, Wednesday, Friday, Sunday it's at DCA). However, the days at Disneyland also coincide with "Magic Morning", which people can get if they buy a 3-day or longer ticket. So the Disneyland one isn't as worthwhile because there are many more people, while DCA is much emptier with only the guests of the 3 hotels. It's been speculated they could move Magic Morning to DCA, or maybe have some days with just hotel guests, but we won't know until closer to the opening. I know a friend and I will be staying at a Disney hotel that weekend to try and take advantage of this extra time.