We did both a cruise and WDW with a child that was a month shy of his 3rd birthday and honestly, it's hard for me to choose.
He loved being on the ship and having the convenience of meeting so many characters there was definitely a plus. There are a lot of activities that they have, even for someone of his age, and he loved the dance parties and such and never wanted to leave the pool deck. However, he also got the same excitement from being in the parks. He enjoyed the rides and pretty much just looking at everything. He did get sick for a little time when we were there but for the couple of days he was feeling up to it, he loved it.
Since you do have an 8 month old in the mix, though, I'd say that the cruise might be easier. It'd definitely be easier to put the child down for a nap since the room is very close and is very easy and convenient to get to. There are also a lot of activities to keep the other children at bay while the parents are separated. While the dinners on the cruise will be pretty simple, the whole sit down thing with your dinner restaurant already determined in advance by your dining rotation, eating at the buffet, at Cabanas, could prove to be a little difficult. We did the tag team thing, though. One person sat down with the child and found a table while the others went and got their food. Also, Cabanas does seem pretty busy most mornings, especially the first day on the ship, but it's not entirely impossible to find a table.