What's your background? If you have a Arch degree from undergrad, I think you'll be fine without another degree. If you don't, and are thinking about another degree, you should get an M.Arch. UD and UP are similar but at the same time very different-UD being architecture at a larger scale and UP being more about policy-making and economics I'd say. The school I went to requires a B.Arch or an M.Arch in order to apply for an UD degree so that pretty much explains the difference. When you say you want to work as an urban designer, what type of work do you imagine?

I have a MUP degree and have been working as a transportation planner (think bike lanes, bus rapid transit, complete streets) for 6 years.

I'd like to work on streetscape plans, corridor plans, master planning, public plazas, public space etc...I'm looking at getting an MUD degree as a way to shift into that realm, since MUD is more design based. There are several programs whom allow planners with strong design background to apply, which I am (Rhino,Sketchup,Cad, etc...).

Clearly having an MLA or March would benefit fit me greater but as a working professional these programs are more time consuming and costly hence my interest in a 1+ year MUD degree.

Ultimate questions though is will it really help me enough to compete with those with March or MLA?

Depending on the practice, maybe.  Most architecture and LA design firms that engage in that type of work would look for MLA/MArch.  The engineering/planning firms may take the MUD, but I see it more commonly as a +1 degree on the MLA/MArch.  Designers like to see studio work I suppose.

This depends on variables, such as software skills and local practice.

By and large, urban designers work in an expanded architectural/landscape arch. paradigm/ practice.  So architectural education is a big plus there.  But I know a few folks w/out architectural degrees working as urban designers.  Urban planning education is key there, but so is the ability to work in graphics software and with a very good visual sensibility. 

It also depends on who's doing the hiring.  Private firm?  Local agency?  The first will usually want an arch. or l.arch. degree.  The second may or may not.

Look through UD job listings.  What are they asking for?  This will tell you much about this issue.

I 100% agree with everything you both said.

From job listings I've seen for consulting firms, some require an MArch while some want an MUD or MLA. Then there are the few that say MUP is ok too, but it seems more rare. I have a pretty solid foundation in graphics but I think not having that design degree is what is limiting my ability to break into that urban design realm i.e. land an interview. I have heard from several working LA and Arch that having an MLA or MArch gives you more clout in the design world. Which, as a practicing urban planner, I tend to see as well.

I'm looking at the MUD as a way to beef up my MUP, to break into that realm. I know others who have been down this path and are now practicing UD, just thought I'd see if anyone had any additional experience in the matter.