My advice - at that price, buy them! I owned a pair and also a pair of Harbeth Super HL5 Plus 40th anniversary edition at the same time. As mentioned, the Super HL5 Plus is the correct speaker in the Harbeth lineup to compare the LS3/6 to.
If you like vocals and listen at reasonable levels as you indicated, and like neutrality as well as great imaging and soundstage, then that's what the LS 3/6 excel at.
The LS3/6 and Super HL5 Plus are more alike than different. The Super HL5 had just a _little_ more detail, but that detail could also come across as being more "bright" than the LS 3/6.
The LS 3/6 do not lack in detail in any way, but listening to them both, you'll notice just a little more with Harbeths. If I were to make a percentage comparison, I'd say you get >95% of the Super HL5 sound with the Stirlings. They're also very nice looking and don't have the fussy grills that the Harbeths do. If you're sensitive to "bright" speakers or prefer a neutral to warm presentation, you'd probably prefer the Stirlings over the Harbeths.