I compared the pros and cons of some countries and the results are in ariezone.volkskrantblog.nl/132465.
Belgium is 7, Netherlands 8.
I agree to the first reactions (automatic sheep & Estrmnky).
Adding some experiences after much cycling in both countries (born in Belgium, living in the Netherlands!).
Belgians are sligtly nicer and more helpful, finding your way.
In the Frenchspeaking part hardly anyone speaks English.
The scenery is nicer (such as has been said) but
- that's only in the south east, the hilly Ardennen;
- and it's more an adventure to enjoy it because it's pretty dangerous on many through-roads. Because of the excellent cycle-path system you'll ride nuch more relaxed in the Netherlands.

Actually I should advice you to ride the Vlaanderen Fiets (= bicycle) route. Together 800 km. but the itinerary along the language-border is very interesting.
Roughly from Tongeren via Leuven, Brussels, Geraardsbergen, Kortrijk and Ieper to the coast.
Or the other way round, depending on wind and next plans/airport to leave etc.
Startinbg in Brussels you may ride the western loop. Then you can also visit Brugge, Gent and Mechelen.
Hae a nice trip.
Arie
Geraardsbergen (B)/Borne (NL)