The 3u diamond paste is nominally the same size as the 3u DMT EE... I'd base the decision on whether you want wood, leather, balsa, or steel as the substrate. With leather, for instance, the edge might be slightly convex; depending on how much give the leather has (strop or paddle?).

I'm with Sham that it may not make much practical difference.

As a general rule, the EE 8k will remove 325 or 1200 striations in 20 to 30 laps; it's very fast. I suspect 3u diamond paste is also very fast. I use the EE a lot, but not with a coticule.