Nikon fever here, I have a d90 with various lens' but like the tamron 90mm for macro shots cause it gives a blurring affect around the edges.
Also have a 3100 for more of everyday use both are great camera's but the d90 is easier for me to use for macro shots mostly cause I have had it for a little while now and am used to it. The 3100 is newer so still getting used to it, the d90 has alot more manual settings and since I shoot full manual I like the d90 better but the 3100 is a great camera for like taking to the shed or museum, etc. So it kinda depends what you want to do with it?
Bottom line is canon and nikon both make great cameras and it's the person that's snapping the photo that makes the difference, you'll have some homework to do like any hobby to take nice pic's.
Also if you already have glass (lens') that's what you stick with in general, the glass is the most expensive part of photography so most pick one brand and stick with it.
Also don't skimp on the tripod, you can get a nice tripod in the 150-200 range that work nicely.
One thing to keep in mind, if you think reef'n is expensive you ain't seen nothin lol!
You can easily drop over a grand just on 1 lens