faster lens or flash?

so i am trying to decide on getting a faster lens (specifically a sigma 18-50mm f2.8) or a sb-600 flash for my nikon d90. i already have the 18-55mm kit, 55-200mm f3.5-5.6, and 50mm f1.8. my wife and i are having a baby this month and i would like to add to my arsenal something that will enhance my ability to shoot my new son (in both formal and informal settings)

any ideas/suggestions? i am really torn!

Flash is cheaper and youll have it forever.

Plus sometimes you just need a fill

Sometimes the flash is what you need and sometimes a lens would be better.

I would also say to remember that you can really push the iso on that D90.
Look at the last pics in this thread at iso 2000.
http://forums.photographyreview.com/...ad.php?t=55805

And welcome to the forums! Stick around and show us your stuff!