Which one is the better CMS and support? My friend keeps debating with me about "mambo" how more people ar writing for it and the support blah blah blah!

Its hard to compare each one has better qualities over the other.

If i were you i wouldn't even thinking of going with FB's phpnuke, if you choose nuke be sure to get daily updated. I would recommend you to go with techgfx's version of phpnuke, www.techgfx.com, i don't know him but he seems way better from FB
I did try mambo a way back, found it's code well written, secure. But it still missing the nuke glorry. Phpnuke is maybe the best CMS out there due to the great community it has, addons, themes and such. So it's your choice. :wink: