Sarah,
I remember your posts from earlier, so I want to qualify my response. You seemed interested in spinnaker sailing, more so than in racing in a one design fleet. If this is not the case, then by all means, get the A Class that interests you the most. I think the Balance A is very pretty, and it seems to be an evolutionary design and will probably be as capable as most A Class.
If spinnaker sailing is a high priority, then I think the Marstrom M18 is the better choice. I think it has been sorted out for using the spinnaker, where I think you would need to figure out adding the spinnaker for the Balance A. The Marstrom M18 is a bit wider than the Marstrom A, but uses the same hulls. I also believe that the mast for the M18 is designed for the spinnaker loads, and I don't think that would be the case for the Balance. I investigated buying the M18 several years ago and the distributor told me that I could easily have both an A Class and the M18, just purchase the M18 and add the purchase of the A Class platform and rig. This is what makes me think that the M18 has a different rig than the Marstom A. The A Class maximum width is 7-1/2 feet, but I think the M18 is 8-1/2 feet wide. Having sailed the A Class, that extra width would be nice, but then again, there is nothing I have sailed that compares to the A Class. It is a whole different world than sailing the H16!
In summary, if you have other A Class boats to compete with, then get the A Class, either Balance, or many others to choose from. The Bimare XJ is manufactured near you also.
If you plan to add the spinnaker, I would start with the M18 and then when and if you want to compete in A Class, get the A platform and rig for the Martrom.
One last thing, never leave your A Class on the beach without it being tied down. It doesn't take much wind to blow it over.