I keep hearing that http://www.nexusmods.com/newvegas/mods/45104/? doesn't only fix bugs but makes gameplay changes. Anybody can name a few?

It seems that most people started using http://www.nexusmods.com/newvegas/mods/51664/? lately. But that one doesn't play nice with http://www.nexusmods.com/newvegas/mods/38285/? (it breaks the All-American Rifle mesh, I think), so I stopped using it.

As much as I like bugfixes, I like nice looking guns more.

The fact MMUE requires a pile of compatibility patches to function properly with some of the bigger mods should be a telling sign. YUP requires none, zipp, nada.

You do not really need either. But if I had to pick one it would be YUP due to the reasons Kirottu stated.

I believe MMUE alters the gear in the Doc Mitchel's house as belonging to the Doc. I believe it was the intention of the developers for us to use that as a means to fund our character through the early stages, especially for players that does not have any of the starting gear pack dlc's

MMUE does indeed flag most items in Doc Mitchell's house as belonging to him, so taking them counts as stealing. I think there may be a couple of items that can be taken without karma loss, including the broken 9 mm SMG that you can fix if your repair skill is high enough.

I think the change in Doc's house makes sense. It is his stuff. Besides, at this point there is no reason not to have the DLC's. They are quite cheap.

Just be sure to install Millenia's Weapon Retextures after YUP. It will overwrite any changes that YUP makes.

I'd agree with you there, but Mission Mojave changes a number of other things in the game as well. It's certainly more than just a bug fix mod, it's bordering on an overhaul.