I'm seeking a directional 2m/70cm antenna with substantial gain over
efficient omnidirectional antennas, such as my roll-up J-pole and my
mobile antenna. I considered the MFJ 1768, but I hear that MFJ
antennas aren't that well made. I considered the Cushcraft A2706S Dual
Band Yagi, but I hear that Cushcraft is notorious for inflated dB

If the key word is substantial then you need to be looking at another
antenna. YOu can not get gain with out boom length. I would not think of
getting an antenna for 2 meters without a boom length of atleast 5 feet. I
would really go to 10 feet or longer if I was going to mount the antenna
outside. Just looking at the Elk antenna I would say that it may have the
close to the gain of a 3 element beam.