Hey
I believe if you want to save money, you should go for Canon as you won't have to buy the lens all over again.
Canon 500D is a 15mp camera, while Nikon D90 is 12 mp
D90's viewfinder is better than 500D's
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Check out these 2 links and compare:
Canon EOS 500D digital camera specifications: Digital Photography Review
Nikon D90 digital camera specifications: Digital Photography Review
Keep in mind that if I had the money, i would go for D90 as its quite an awesome DSLR. And i just compared D90 with 500D and no other Canon DSLR.
Hope that helps
