loading supervisor and ramp agent are working together on a specified parking position from arrival to departure of an aircraft, so looking on a hirarchy tree, the ramp agent and the loading supervisor would be on the same level but with different tasks.
the ramp agent organizes, mostly via radio, the complete turn-around of an aircraft - he is the link between an airline and the airport. the ramp agent is responsible for right in time passenger transportation from terminal to A/C and back, for ordering ground power units, aso...
during this time the loading supervisor is responsible for the loading of cargo and baggage.
experience in aiport operations is liked to be seen for both jobs, but not mandatory (still speaking about VIE/LOWW but we all are working accordingly to ICAO, JAA, aso standards).
compared with flight attendents i would guess loading supervisor and ramp agent, in my personal opinion, is higher qualified personell - just think of aircraft weight and balance sheets and it's calculation, etc.... but it's another bunch of jobs so you shouldn't compare them
training for those ramp jobs normally will last at least between 2 to 3, up to 6 months. ramp agent have additional courses to attend than loading supervisors, so a logical career step after being loading supervisor is ramp agent.
Chris