very personal view and just generalized impressions -> I don't feel up to being able to justify a full tilt pros/cons...

-slightly more varied countryside in belgium (in terms of hills etc.) although the dutch would prob. love to argue that point
-more water/coast etc. in (literally!) the netherlands if you like that (like me), a tour around the Ijselmeer through all the quaint towns is real nice
-very interesting historical towns & cities & architecture in both so no advantage to either imho
-urban city flair: on that one note, the dutch beat belgium with "the 'dam" hands down, sorry
-how's your french or flemish? Belgium leans more towards those while the Dutch are often near accent free

-great time of the year to go btw
I've been to both repeatedly, and like both very much. Hard to call, sorry