Paul, I do not have experience with aftermarket harnesses. I always make my own harness and tailor it to the car. Most owners do not have the capability or electrical background to undertake such a venture. However I have see lots of cars with aftermarket harnesses. It seems to be a trend these days. I have yet to see any aftermarket harness that has lead lengths correct for the ideal fit regardless of brand. But I admit I am picky on this issue. All I have looked at had better connectors than Lucas and all appeared to have acceptable workmanship.
I’d vote for the less expensive harness. If you have the ability to solder wires correctly and have some basic understanding of how to trace a wire to a specific point, you can fix any problems, should they arise. Perhaps some one has some specific issues to point out for a particular supplier. I do not.