Pro - Planer allows you to fight just the fish once hooked. Con - They also track funny and can cause tangles. Heavy lures at slow speed seem to trip them constantly (ymmv). Planers run at set depths leaving large areas lureless.
Pro - Inlines are more variable and you can really pinpoint your lure depth. Inlines are less affected by speed. You can easily cover the water column. Con - You fight the weight with the fish and with heavy in-lines, can't even feel smaller fish..
In addition to everything above, diving planers give a bait different action, and/or perhaps provide an attractor. You will catch many more fish on a drone, clark, or squid spoon behind a diving planer than behind a sinker.