Cameras are tools to Photographers, like Brushes to Painters.
Neither having Picasso's brush doesn't make you Picasso; nor having Ansel Adams camera will make you Ansel Adams.
Frankly, I find myself recommending and liking not only the brand I use, a photographer and his tool are a matched pair - you get acquainted to a tool by holding and trying it out. If it feels good and works in your style - it is a good tool. Digital cameras are all great machines - if you look back just a few years ago they all have improved tremendously - we are in Digital Photography paradise.
However, many people would treat these tools as objects of vanity - especially if it costs a considerable amount and people take pride in the ownership of one. Some do get satisfaction by saying that others aren't good etc etc - but they should know the phrase: "Snubbing out other peoples' flames doesn't make theirs burn more brightly." But that's exactly what I think your friends are doing to you regrettably. :sweat:
Your D90 is a fully capable and well rounded photographic tool. I'd advocate that you choose bro Catchlight's suggestion #2... they'd shut themselves up automatically.
If you have skill and the goods, you will be respected for your skill and vision in photography, not being part of a clique who uses the same equipment.
Don't worry, be happy, shoot more! :lovegrin: