if you give 700 you will be certified for 8.1 and in the future if you want to be certified for 9.1 you'll have to give another upgrade exam (upgrade from 8.1 to 9.1)...
As such the basic core stuff remains the same in both the scenarios....so both the exams will help you immensly
9.1 will have more improvements and functionality which you'll miss out on if you give 700...
Personally I gave 730 even though I work on 8.2... as it allowed me to explore additions / functionalities in 9.1 and be up-to-date.
For you, I personally feel you need to take a call on what will best address the reason why you are giving a certification exam in the first place.