I would just finish out the other exam. There is not a whole lot of different aside from updated material. I'm not downplaying that at all and keeping your certifications up-to-date is important. But if you have already passed the first 600 level exam, just take the other 600 level on. Then in a year or two if you want to update the A+ you can.
This way you can say you are certified, your certification will be 'up to date' through next year i believe (that is when they stop giving out the 600 level exam) and you can tell employers you are A+ certified.
The CompTIA exams don't expire and most employers only care if you are certified, not certified with the latest exam.