I flyfish the mouth of the Niss quite alot over the years.
Either rod would be appropriate depending on certain variables.
If you are fishing smaller flies like sandeel imitations or small clousers and decievers the 7 wt will work fine.....If it is windy or you are fishing larger flies go with the 9 wt.
Also if there is a chance of larger fish the 9 wt is a good idea so as not to tire the fish too much. This last consideration is very important remember a 9 wt rod is light tackle and plenty of fun.