The Intercontinental is a much nicer hotel, though entry-level rooms might not impress. The club lounge is comfortable and has quite spectacular views, but it's one of the rare places where the catering isn't all that much better than at the Hilton.
Both are convenient for Taksim, but the IC has the edge here, with the metro only a very short walk from the hotel.
But it's not much fun staying with no status leverage at all, so perhaps you should go for the Hilton and keep your fingers crossed for the upgrade fairy to her work.