Thanks for your interest in the Nice Memberships extension. I appreciate it.
One thing that I want to point out is that the Nice Memberships extension uses Joomla's registration form. So, the extension has little control over registration forms. It was designed not to have this dependency to provide you with more flexibility in its deployment.
That said, the Nice Memberships extension is equipped with the ability to automatically register users, after they have made a successful subscription purchase. So, rather than having prospective member's click on Button X or Button Y and then register with form x or form Y, with Nice Memberships they click on Button X or Button Y and immediately go to PayPal to purchase the subscription. After which, their account will be automatically created and they will be placed in group X or group Y, depending on what subscription they purchased.
~ Best regards