General understanding is that in most cases status first, then fare class. If you're E50K on a Latitude fare, then your best bet is to find a flight where R space is open and upgrade it before the SE window opens at T-7 days. Otherwise you're competing with everyone else.

P25K can upgrade Latitude e.g. 3 months out, but at the gate, they'll be behind E35K, E50K, E75K, S100K on any upgradable fare (including Flex).