Yah, there's around 150ms-1 boost speed difference, more or less - I'm out in the black at the moment so can't be certain but AIR my Clipper makes around 590ms-1 under boost and my Courier makes around 740ms-1 if I take the weapons off. Both ships can reach their full potential speed even when using heavily tweaked dirty drive tuning, which is a definite plus - that's harder or impossible on some ships at present, especially with any kind of generally useful build, so the "paper" speeds turn out less than the "practical" speeds..
The Clipper was so far under the optimum mass for the thrusters that I was able to load it up with 150 tons of cargo and
still hit over 590ms-1.
As far as searching planets go, I'd take an Asp over the Courier any time (if I wasn't worried about being attacked), even though I prefer the Courier as a ship; the much better (i.e. it
has a downwards view) downwards view of the Asp means you can fly in the cockpit view and scan the ground, while if you want to do that in a Courier you either fly upside down or go outside into external view and lose all the feedback from the instruments. Similarly with other Lakons, and if I remember rightly the Vulture has downward view panels too. Some ship like that that you wouldn't expect, anyway. Think it's the Vulture.