When evaluating a headset, you look at dB of attenuation, both passive and active.Generally, passive-only headsets have more insulation than ANR headsets, and not surprisingly, do a better job of passive attenuation at the high frequencies.
However, an ANR headset really excels in the low frequencies, say below 200 Hz, as compared to a passive headset.
So, it really depends upon what kind of cockpit you're going to be flying in - high or low frequency noise?
I've tried the Headsets, Inc kit, and the David Clark ANR, and I wasn't impressed by either. The conversion kit requires that you remove so much insulation, the higher frequency attenuation is noticeably inferior.
I fly in very noisy cockpits: Maule, Pitts, etc. I use the Bose Series 2, which has superb active and very good passive attenuation for an ANR headset.
Unfortunately Bose doesn't make the series 2 any more, it has a completely new version (series X?) which has hardly any passive attenuation compared to the series 1 and 2, but is much lighter.
People complained about the weight of the series 1 and 2, I guess.
One thing I do which improves the headset performance, is to keep replacing the ear seals and I also have a chin strap which is admittedly for negative G, but also improves the seal when I'm at +1 G, too.
I should mention that I was instructing on a Cessna 180 recently and the guy had Lightspeed ANR headsets. I forget the model (25?) but they worked very, very well. Probably almost as good as the Bose. I'm not sure they'd be as durable as the Bose - mine have lived a very hard life and still work perfectly - but geez, for the money they sure sounded good.
Lightspeed has a new, low-price ANR headset out (cross-country QFR ?!) which is what I'd look at closely if I was buying another ANR headset on the cheap - $250.