I think it has been left up to you simply because there is so little difference between them in terms of effectiveness or side effects. The saving grace in your specific situation is that if you start on one and find within the first 3 months that you have serious side effects or it isn't working, you will be able to change to the other. After the 3 month mark, you cannot have abi after enzo has failed or enzo after abi has failed.
You may have to toss a coin :-/