I have to fly to SFO on 8/9 - there are plenty of options through PHL or CLT, but I'm wondering if anybody has any advice on which route to take to get a better chance of an upgrade. I'm checking SeatGuru to see how many F seats are left, looking at seat maps to see how many "Preferred" seats seem to have been taken so far, but it seems to be a bit of a toss up ... any anecdotal advice about which connection tends to have more availability? Is it better to take an early or late flight on a Monday? I don't fly this route much so any advice is appreciated.

I'm platinum now, will possibly be CP by 8/9, depending on how long it takes to change my status from a flight the previous week.

Thanks much.