I couldn't read all of the posts but I see it has generated some emotion. I reread the OP and here's my opinion (if you are even still interested, LOL). A few years back, when our kids (two cousins) were barely 2, we won a trip

to WDW to see the NFL quarterback challenge. We went, leaving the two kids with their grandparents. It was not easy as we don't normally even get a sitter to go out for the evening. But, as you said, the kids would have definitely slowed us down. (Sorry, everyone, I don't mean that in a bad way, it's just fact). Normally, we would have been okay with it, but since it was a planned trip, with activities with the quarterbacks, etc., it would have been very difficult to manage with the kids. It was a short trip (4 nights) and they had each other and lots of fun with their grandparents. We told them it was an adult trip and they were too young and they accepted it. My daughter still remembers it, but not with any sort of emotional distress.
After that trip, we all went again WITH THE KIDS when they were just barely 5. This is a wonderful age for Disney. That being said, we now have another DD-barely 2, and I wouldn't dream of going to Disney with the older DD, now 8 and not the younger one. I see it completely differently when it comes to taking one child, but not the other. My older DD wouldn't dream of it either. As much as she enjoys being with me or DH alone, the idea of leaving her baby sister at home would assault her sense of fairness. I believe this was the crux of your question. And, I do tend to slightly agree with those posters who were concerned with the message that it would send to the older child. It seems to devalue the baby, in a way. Sorry, again, this is just my opinion, and it's worth what you're paying for it.
I think that your baby may not slow you down as much as you might think. Your older son, at almost 5, is still kind of young for a lot of things at WDW.
We skipped, or did "baby swap" for all of the scary or intense stuff when our kids were that age. Also, you 'll need to have a stroller for your older DS anyway, even though he never rides in one anymore at home. Disney is not a walk in the park (

) Sorry, couldn't help it.

Also, if you go back to the hotel for a nap, the older DS can have one-on-one time with either you or DH at the pool each afternoon. At age 4-5, the pool time is often just as good as the park time. And the parent who stays with the baby will get a nap too (with no guilt - what could be better?)
Anyway, you will decide what's right for your family. I agree with your thought that, if you decide to leave him, it would be easier on everyone if he was 14-15 months, rather than 18 months. You don't want to hear him wailing, "me too, me too, me too" as you go out the door. It will break your heart.
Good luck to you.
