Yeah for the closest possible comparison in your scenario, you want to do a 3 year reserved with heavy utilization.
When you reserve an instance, you are committing to a higher upfront in order to get a lower hourly for the reservation period. The low/medium/high utilization is sort of a knob that allows you to further control this upfront vs. hourly cost. With high utilization you have the highest upfront, but also the lowest hourly cost. If you are planning to run the server 24x7, this will also give you the lowest total cost.
Not only that, now that they have that marketplace going you can pawn off your contract unto somebody else if your needs change.
I'm not sure how liquid it is yet. You could also equally go dumpster diving for unused virtual instances, say, if some startup goes under and liquidates.
