I would not consider that an Exadata specific question. If there was a valid reason why you have had a Blocksize of 16k before (which is doubtful, especially for an OLTP system), this reason will still apply on Exadata. Generally, I'd recommend to use the Blocksize 8k unless you have a very good reason to use another value.
The cons of using the non-default block sizes usually outweighs the pros. Remember that there are blocks-size specific bugs (mostly for 32K however) so every time you're running something different from what other people are (mostly) running your risk increases.
The best way is to test both and then see if additional benefits from running 16k (if there are any) are worth it.
However, be careful in one regard -- the vendor might have legitimate reasons for recommending 16k block size. For example, they might use index keys that would be otherwise too big for 8K blocks or using smaller blocks might introduce unnecessary row chaining/migration. Though I'm deeply in doubt that there are many vendors around who write software for Oracle DB understanding any of that ;-)