Yep, both nutaku.net and nutaku.com are valid. You can use the same account in both places. And you can continue your progress from whichever place you want, (nutaku.com or nutaku.net).
Nutaku.net is the adress where you will find adult (and english) versions of the games (example nutaku.com has the non adult version of Flower Knight Girls and nutaku.net has the adult version).