I got a masters in planning, worked for several years, and then eventually went back for an M Arch. If you want to design work, don't kid yourself, even if you area able to get enough skills to draw master plans and diagrams, your career options will be very limited if you don't just get a design degree. The fact is, an MUP, like many masters degrees, is a piece of paper and an opportunity to build some valuable relationships. It is similar to an MBA, you learn a few applicable skills, but the degree really is not about the skills, it is about a rite of passage to enter the profession. By contrast, a studio-based design degree, an M Arch or an MLA, is a REAL degree - you leave the program a COMPLETELY different person than the one you were when you entered. The degree is not a replacement for your own creativity (as others have noted), you must have that as well, but you will discover that in the studio - you learn about yourself and you develop your design ability. To be quite honest, in contrast to what others have said, the standards/dimensions are not the most important thing you learn in design school, nor is it the software and graphics skills (although both of these are certainly essential); design is a set of "soft skills" that you must develop through a long-term commitment to the art.
Most planners who claim to be design-minded, have really just memorized a few principles and best practices, such as street-building relationship, scale issues, and maybe some street width dimensions. There is a reason that the New Urbanism is so popular among planners - it is a set of easily understandable rules that non-designers can memorize. However, knowing best practices is NOT the same thing as design. Design is about taking a set of conditions and MAKING something out of them. In real design, there is no textbook, there is no template, there is only your ability as the designer to solve the problem at hand. And if you are good, you can solve the problem AND enhance the experience of the user. You can make place. You can make poetry.
I wish I could go back and save the two years I spent in planning school. After years of school and working in the field, I am confident that the absolute truth is this: if you want to be a DESIGNER-designer, get an M Arch; if you want to be a PLANNER-designer, get an MLA. City planning, and certainly urban design, IS a design discipline; however, city planners are not typically designers, and this is a major problem. This means that most of the professionals who are in the field are not actually equipped to do the job. It is not the architects (or really even the developers) who are to blame for the lousy built environment in our country, it is the PLANNERS themselves, and the wide-spread professional malpractice on the part of the city planning profession who are not trained to do the job they are paid to do. Planners, in general, are spatially incompetent and design illiterate. This comes as a result of the fact that city planners are not trained in city design and city building. Landscape architecture absolutely can (and quite frankly should) replace the profession of city planning within the next 15 or so years. LAs are far more equipped, due to their design training, to deal with the planning problems facing cities and regions in America than the so-called planners.
The short of it is this: JUST GET A DESIGN DEGREE!! It will be well worth the time and energy you invest in it. If you just put in the time to get a real design degree (not MUD, not MUP - unless it is a dual with MARCH or MLA), you will never need to be self-conscious about your skills or about the value you bring to a project. You will have wide career opportunities (in consulting firms, in public sector, on your own, as a planner, as a designer, etc.) Plus, studio design education is so much more enjoyable than planning education! I loved architecture school! It was tough, but so incredibly rewarding. By contrast, planning school was a money and time sink without the payback. JUST GET A DESIGN DEGREE!!!! (I wish I had listened to the people who told me that when I was starting planning school....)
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