My opinion is that .nu is not a .com and most people in North America wont know what .nu is. You will get people that wont remember .nu as they would remember .com name.
You have 2 excellent names there, girls.nu and my6.com is very short and easy to remember, even though girls.nu is more oritented to what your selling, I would go with the .com name.
I am sure everyone would trade with your domain it does not have anything to do with that, its the fact that you want people to remember your domain and come back to .com names.
That being said, why dont you use Platinum Bucks sites as your sponsor for your site. Conversions 1:50+ on high quality traffic.
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Platinum Bucks
Cash4Blind
VP, Marketing & Operations