B fares where there is AN inventory available or a Y fare when any seat up front is still for sale = instant upgrade. Everything else is a crap shoot.

While you will do slightly better with the higher fare, the fact that you are at the bottom of the list anyways is really what is going to hurt your chances. I wouldn't buy from V up to H. I'd either buy the instant upgrade to be in F or the cheapest and expect to be in Y.