I am not a realtor, just a resident of German Village.
To answer your questions I would say it depends on where you are moving from. We moved here from San Francisco after living in Chicago, New York and Boston. German Village is safe. Columbus is by nature a very suburban city, it just has been since the 50s and is ingrained in the mentality. Not bad or good, just real and you will have to deal with that when/if you move here.
Because of that, neighborhoods dont change as quickly as they do in other cities, so I would suggest not to live on the edges of neighborhoods. Though tempting because you think there may be more appreciation, it doesn't happen often.
The neighborhoods that you mention are both great. I would also suggest you look at the Short North if you are coming from a major metro area. Everyone has an opinion, mine is that if I was a professional out of college I would want to be in the Short North. Great restaurants, a lot of activity, a real city feel relative to the other neighborhoods you mention. I am 32 and have a kid and love German Village because it is walkable, great restuarants and a great park.
Long winded answer, but generally, I think you should look at German Village and/or Short North. Merrion Village is a bit on the edge and likely with the economic downturn wont continue its recent upsurge. My thoughts...good luck