I agree with the OP: I don't like paying extra for a ticket that I have to wait until the upgrade window opens up (or even worse - the gate) to see if I got the upgrade. I've no problem waiting if I've paid the cheapest price.

I haven't looked at the change fees, but what about booking the cheapest T+ fare then phoning 4 days before to upgrade to a T+ M fare if there's space available? If the change fee + fare difference brings it up to the cost of a Latitude fare then I'd book that now.