Mass is good, i have the 100w regular one (with all the controls on).
There was a big Harmony Central thread where some guy compard all the attenuators together.. he found the Mass and the Hotplate were pretty much the same as far as altering the tone (eg they dont change it much even on max attenuation).
if anything the Mass was 'better' as its switchable OHMS (hotplate isnt), has more tweak controls (tone stack, high\low filter + 3 way treble boost\cut) has 2 speaker outs and a DI line out with tone controls and costs around 2/3 the price of the hotplate.. well actually my Mass was $130 /£70ish inc postage (no VAT) whereas the the hotplate is £160-£250 depending on what Ebay holds for you!!
The KOCH loadbox is meant to be good and tweakable but expensive, as is the Dr Z Airbrake.
The Marshall PowerBrake is by far the worst as far as user opinions go.. badly 'sucks tone' and can overheat and fail.. and doesnt offer as much -db range as the others.
Weber Mass everytime for me.. Maybe the Koch one if i ever had money to burn (or a London Power scaling setup installed by a tech..)
www.harmony-central.com is the best place to start for opinions.. but take them all with a grain of salt (as they say) ;)