We have both Impero & AB Tutor
We have been using AB Tutor for 3 years now as we found it to be the most cost effectivve classroom desktop management solution available for a network of over 700 systems with 15 suites. The licencing based around Tutor licences and unlimited client is by far the cheapest solution for us.
We started looking at Impero along with Securus (as these were the two recommended in the Edugeek forums) as we wanted a network activity monitoring solution and not necessarily a classroom desktop management solution. Like most we struggled with keeping up with the filtering of the hourly creation of Proxy sites. We just didn't have the time to be monitoring all our students activity and disabling the sites in our Smoothwall filter or at Grid level filter that they were bringing into school. We needed on screen monitoring. Both Impero & Securus provide us with this and Securus has just resleased flesh tone recognition which was very tempting but the reason we chose Impero was that Securus was still a retrospective action system which relied on a technician reading through logs daily and disabling accounts. Impero developers assured us that they would consider adding to their policy structure the functionality of when a user breaks a particular policy their internet is disabled wherever they log on to the network and can only be enabled by a console user. This means in one fell swoop we would cut proxy site use as as soon as a few are caught and loose access to the internet for a period of account (depending on their previous offences) then others will not risk it.
At the moment I don't know how much of this is a sales pitch but we payed a price for the licence now that enables us free upgrades to version 3 which would cost us double to buy.
The fact is that Impero and AB Tutor functionality overlap quite a lot. If you only had the budget to buy one and you have no desktop management solution at the moment I would advise you go for Impero. If you can afford both AB Tutor is far more polished after the recent upgrade and is far more intuative for classroom teachers to use.