Is it possible redirect user to different Nintex forms by a users choice of value from a dropdown that exists on a Nintex form?

Yes this is  possible, but you will have to use JavaScript to perform the redirect (especially if you have more than one page redirects depending on the option they select).

As an alternative you might be able to try something  like  suggested here:  

This is a sample with JavaScript which maybe helps you. It is still one form but you can have different pages.

http://www.sharepointpub.com/jquery-ui-tabs-and-nintex-forms/

could you perhaps explain what you want to achieve instead of direct technical question?

maybe we could advice some better/easier approach...