Ported vacuum advance does not take effect until the throttle disc has passed over the port in the carburetor throat, which is above its normal at rest idle position.
Full time vacuum advance is always exposed to engine vacuum since the source is below the throttle disc, so it's always present.
These differences change the idle characteristics depending on the construction of the vacuum advance canister ("can"): how far it advances the spark and how much vacuum is needed.
For many engines different cans are available or can be modified to work.
Engines with low compression or large overlap may have a shaky idle with full time vacuum, you really have to try both to make an informed choice.
