I have a D5100 that I just bought a few weeks back as well as a D200. I've read all the replys and it basically comes down to your budget (Doesn't it for most of us!).
Lucky for me I have an SU-800 & 3 SB-800s which I use with my D200. I'll be able to use my SU-800 on the D5100 and still have capability of 3 off-camera flash units.
If it was me, I'd go with the SU-800 and then, if I had to due to budget, get only 2 flash units such as SB-600, SB-700, etc... Once you get some sales, you can diversify by adding another flash.
Don't rule out the SU-800 for outdoor use entirely either! You have to pay attention to what you are doing in aligning things up but I've used my setup outdoors lots of times for nature photography in my back yard as evidenced by this pic in my gallery.