I like to think that it all stems from the different devs' philosophies, or approaches, not sure of the word.
BTW, the following is just my opinion based on using some stuff. Your views may vary and are every bit as valid as mine if not more so.
Mike has always been about tweaking for his vision of a max Sense performance rom, yet still providing flexibility via auxiliary packages/mods. If you like getting your flash on, you'll love being a Meanie.
The Viper4G team is set on basic stock, but with the easiest path to customize look and feel, and perform basic maintenance (cache cleaning, zip aligning, permissions), and cleaning out hidden cruft like temp files on reboot. If you want a conservative Sense approach, with few updates to mess with, but better than stock performance with the fun of themes and no steep learning curve and no digging through forum threads to see if something works, Viper4G is for you.
If you want a less conservative Sense approach with the bloatware fully removed, but not the edge for the Meanies, then CleanRom is probably a choice to consider, despite it being less popular in our subforum. That, like flipz Fresh Rom, deserves attention.
And then there are the high performance, other hybrid / partially de-Sensed roms like freeza's to consider.
Hmmmm....
I started out wanting to make comments on two roms, and got carried away.
Guides, if you want to start a thread to describe roms and want to pull this out of here and use it in any way, including kanging so we can delete this post as off-topic for the Meanies, fine by me, drop me a line for the delete.
Because we haven't even gotten to AOSP here!