What's the difference between an upright canister stove and an integrated canister system?

Upright stoves like the MSR PocketRocket 2 or SOTO WindMaster screw on top of a fuel canister and accept any pot, making them lighter and more versatile for actual cooking. Integrated systems like the Jetboil Flash bond a heat-exchanger pot to the burner for faster, more wind- and fuel-efficient boil times, but they are heavier, more expensive, and most don't simmer well.