I'll agree with pretty much all the things said above, which means I'm not really helping you make a decision.
The comment that "sloops sail more efficiently" is certainly true on the wind
, but off the wind
you will be able to balance a ketch
better. If you are good (and patient) even to the point of self steering
even on a beam reach or a little below.
And you have the options to carry a lot more sail. A ketch
flying an asymetrical and a mizzen staysail will leave it's sloop
rigged sistership way behind in light airs off the wind. In this case the definition of "off the wind" varies with wind strength. the lighter the wind, the higher you can point with this setup. At 5-10 knots true you might be able to reach 50-55 degrees.