I'm torn, but here are my thougts:
1. Do you have time to see how they do with the April trip? That will tell you a lot. If they are not thrilled when they get there, hold off a bit on your next trip.
2. Will your kids be able to be on vacation that long? I've seen many children not be able to handle being off a normal schedule for that long. Maybe yours are better, maybe you can maintain a decent schedule while at WDW, especially with such a long trip and no need to rush? Just something to think about.
Personally, I would go for 2 trips to make my time between shorter. However, I've been to WDW less than you 7YO, so I'm nowhere near burnout!
One last thought, your kids lack of excitement may not be burn out. They always know they're ocming back and roughly when, right? So, maybe, unlike those of us who have gone through the "I'm leaving and don't know when I'll be back trama", they just know they'll be back. Not that they won't enjoy every minute, but te y just don't have withdrawl.