I just tested the c3 and have been using problend 600. Let me look at my notes.
What they have in common -- both give me relatively smooth tops. However the c3 gives me more dips near the wick. [I'm comparing these to the lumpy, crater and sinkhole nature of 464 and 444) Nothing major. c3 is a single pour wax if your ok with a slight dip. Problend is as flat and smooth as you can get with a single pour. Huge plus.
I think c3 lasts a bit longer than the problend.
Problend is soft and somewhat sticky and comes in small slabs, c3 comes in flakes so it's easier to deal with, less messy, in terms of handling and measuring wax.
The c3 doesn't burn as clean as the problend. Problend has a consistently clean burn. C3 has personality issues...I dunno...throughout my burn test it kept changing dramatically. However c3 finished clean...odd.
Problend is more stable, more predictable, more fool-proof...imo
Note: I tested in small 4oz tins