For the sake of everybody under 35 or so who has never seen us play there, I really hope it is at Wembley should we make it to the final.
Generally though, I think holding play off finals at Wembley has done an awful lot of damage to the cup competitions. The whole point of reaching a cup final was for the incredibly rare opportunity for most to play at or watch your club at the national stadium. Now that up to 16(?) clubs are doing it every season, it means very little if there isn't the prize of promotion at the end of it.
Wembley has to pay its' way, but when it's all about finances it takes something away for what should be very special.