First of all: the "slightly better than the client" is always a good guidiance.
Then 2ndly: What matters is a combination of what the client/company/your manager expects and what you feel comfortable with. Personally I would say as the other response on this site - better to error on the formal side; suit/shirt/tie is always a good starting point.
Plus a practical consideration from my personal experience ("you know you have been in consulting too long when optimising you morning clothing procedures ;-): Suit/Shirt/Tie may be slightly more formal - but it is sooo easy - if you are trying to dress "smart casually" there is a lot of choice each morning - it is a lot easier to limit the choice to "which suit, which shirt and a matching tie" ;-)
- and welcome on board; i hope you - as I did - will find that PA is a nice and challenging place to work.