It depends a bit on which PHL flight you're on. Arrivals lounge might be quite busy coming off the BA66 06:35 arrival, I tend to think that's about peak time for the showers there. Arriving off the BA68 at 10:00 arrivals lounge will be much quieter.

Of course, you may well save time going landside even if you immediately enter security again, so checking the arrivals lounge shower situation is almost no extra time compared to remaining airside and going through Flight Connections to GF.