Why not a Masters in Urban Planning instead? Should make you more qualified/attractive than being yet another arch undergrad. Also, you're fully able to work in an architectural office as a planner w/o an arch degree and get to design the urban & planning elements; two partners at my previous office were planners.

My two cents - if you get an architecture undergrad you're more likely to be put to drafting/technologist work than any urban design, though do have better job security through your versatility. Unlikely to help you get hired over just getting some experience or going for the masters though.

If you are interested in designing urban spaces would not landscape architecture be a better fit than just architecture? 

I've seen urban planners hire arch students and recent grads to produce drawings, renders, and graphics for them.  The high level content remained squarely the in the hands of the people with urban planning degrees.  The arch people were often temporary contract employees or low-level staff at best.

If you are in the USA, it seems like a Masters of Urban Design from a prestige institution like at Harvard or University of Michigan would be the place for you.   It seems like the best fit towards your stated goals to continue your studies and be positioned for employment.

Unfortunately the professional ranking of a Registered Architect typically out ranks a planner / urban designer / landscape architect.  I say typically but usually the Architect will take on the professional role of the coordinating professional and there by leads the team of sub consultants which may include all of the above or may not.  If you don't ever care to lead a project team and want to play a supporting role then the Urban Designer / Planner role is great and depending on the nature of the office and project type you may find yourself with really good opportunities but from experience I would say the Architect is usually the team leader.

In a word "no" - go for your American Institute of Certified Planners (AICP) certification which involves an exam much like the ARE - this is administered by the American Planning Association (APA) and is a future certification required for some governmental and private sector positions - I took Urban Design at UNM from Morton Hoppenfelt (Columbia, MD & Summerlin, NV) who was the dean at the time and a licensed architect - search "Morton Hoppenfelt" for much more information"  - the first day of this graduate course he stated "if you want to understand urban design you first must understand architecture" - as you know buildings are the building blocks of communities (cities) as well as other elements of the urban fabric - wide-area planning is so exciting with secure employment opportunities ...  

Me once again - this is some of what urban designers do - this is from a job description from this website ...

Urban Design: Develop comprehensive urban plans, zoning regulations, and land use proposals. Integrate sustainable practices into urban design and planning projects.

• Conceptual Drawings & Renderings: Create detailed drawings, renderings, and visualizations to illustrate proposed urban layouts, streetscapes, parks, and other public spaces. Ensure designs are visually appealing, functional, and adhere to project requirements.
• Site Analysis & Research: Conduct site analysis and research to assess land use, zoning regulations, environmental impact, and community needs. Use this data to inform planning and design decisions
• Collaboration: Work closely with architects, engineers, landscape architects, and other professionals to ensure cohesive and integrated urban planning solutions.
• Regulatory Compliance: Ensure all planning and design work complies with local, state, and federal regulations, as well as company policies.