It depends on where I'm riding. I don't notice it being too tall when I'm riding it, but then can get myself into more trouble on it than I do on the TTR. It's hard to explain. I have got the longer dogbones on it for me and a lighter spring (stock is 5kg, I've gone with 4.4kg).
Have a look in the for sale forum. If you know it's really the best bike for you, go for it. You may never pass it on to your wife, but I'd guess you'd be better off with the KDX. You can do all sorts to suit your riding style/preference, so could alter the bike if you do decide to get your wife to try it (i.e. stock pipe over an aftermarket one is a huge difference).
As for new v used, the KDX hasn't changed since 1996, except BNG's (bold new graphics).
Hope that helps some - I just don't think you or your wife would be happy with a KLX125 for very long as neither of you are small folk (my husband is 5'11" & enjoys riding the TTR, but that's more as a joke & never for long). He steals the KDX when I'm on the 125 & it's become another bike for him (stock dogbones go back into it).
Whoops, forgot to mention - until I got the TTR, I used to follow kids on a LT80 (quad, slow rider) and XR80 (or lead, whichever). The bike handles this fine too (got a PC pipe & FMF muffler, rad valve, modified airbox lid) but is fine handling the quicker rides too. The one pain is I've taken off the sidestand due to it being bent because of the lowering of the bike. I've changed the spark plug a few times in the 3-1/2 years I've had the bike - fouled it once, replaced twice due to wanting a fresh one for a race. The time it fouled was due to a rag being left in the carb (both my husband & I thought the other had taken it out, of course as it's my bike I get the blame totally for it).
Depending on how confident I am of the kids riding, if I'm following them, I either let them go a bit & then chase them down or go right behind them - same as leading. I don't have my own kids (thank god), but ride with friends & theirs, hence I ride with different abilities & ages & bikes.
Michelle