I'm also in the exact same dilemma, but with CMU MCDS vs Brown MS CS (UCSD and Columbia are actually the only two school that haven't given me a decision yet). I share the same concerns as you both but I don't have any background in Data Science. I'm very much undecided but here are some of my thoughts.
I think a lot of employers aren't going to be familiar with specific programs and value your degree based on the school without factoring much into the program.
A lot of SWE roles like to slap the Big Data/Data Science buzz words into their job descriptions whether it is actually true or not. When I interviewed for my current role, which is a SWE, they had Big Data in the job description. Having worked there I can say it has very little to do with Big Data, but knowing the people in charge of hiring they would look favorably on people with Data Science experience.
A more traditional MS is probably more highly valued in academia, because most of the programs leave room for or even emphasis research, if you want to be in academia this is probably the better choice.
On the other hand the MCDS program has a 4 semester track where you take a lighter course load to leave room to do research.
As meesuni88 said in their post, I'm also very concerned that the specialized nature will force me down a specific path without leaving room for exploration.
Many traditional MS CS programs allow you to take a much broader set of courses and you may discover a passion in an area outside of data science that you didn't know you had.
On the other hand many MS CS programs make you select a focus early on and you end up taking the majority of your classes in one specific area anyways.
For employers, If you don't have a PhD then a professionally focused masters (like MCDS) may actually be looked on more favorably then a non professionally focused one like an MS in CS.
The employment destinations of MCDS graduates are very impressive: https://www.cmu.edu/career/documents/2017_one_pagers/scs/Combined_SCS_MS_2017_Final.pdf. Many companies encourage mobility and if you think Data Science roles aren't for you, they may pay to train you in a different area and allow you to switch roles. Essentially the MCDS program may be good for getting you a foot in the door in a top company.
I'm not sure if this one matters but I've heard from a couple people that attended different programs that CMU is very intensive and very difficult and that you should be ready to be stressed/anxious for the next 2 years.
Ultimately I think it's too hard to say no to the #1 ranked school.