Since the OP lists Delta on his profile:

ExpertFlyer shows the medallion/pmu/miles upgrade buckets of V,G,X, and Z, which KVS does not. So if you're mostly interested in upgrades, ExpertFlyer may be the better product.

Conversely, KVS shows award availability for SkySaver and SkyChoice seats, which ExpertFlyer does not. So if you're mostly interested in award tickets, KVS may be the better product.

So if you're interested in both of these kinds of seats on Delta, you may need a subscription to both.