I would add that the Hilton Diagonal, while nicely refurbished, still has the exec lounge in a hole in the middle of the hotel - so no views, and not a particularly welcoming place to sit and have a few drinks. If you end up staying there (and absent the lounge, there's no reason not to), I wouldn't pay the extra to get lounge access, but rather go and have some drinks out and about. The Diagonal Mar, on the other hand, has a nice lounge with a terrace and views... but there is a thread somewhere recently about the offering being downgraded.

As a Gold I was upgraded to Exec floor at the DM in November, and not upgraded at the Diagonal in June. I think on a longish stay in August you would be lucky to get upgraded, but personally I'd save the money, take the chance, and enjoy some lovely pavement cafes and bars instead.