I have a Midi, 2xCT26, and CT36 AC.
Yes, you can switch off the autoclean function if you are not using this vacuum for drywall sanding. On my unit, the switch has 5 positions: Off, Auto, Auto with AC, Manual, and Manual with AC. Controls are slightly different on European version, but as far as I know, the functionality is the same. You can download the manual from Festool's web site in your country. (it would've helped if you had indicated in your signature where you live)
It comes with 1 micron filter (compatible with autoclean function and drywall dust), but one can swap it to the CT26/36 HEPA filter (if you are not located in North America, you have a different gradations of filters, we only have HEPA which filters down to 0.3 microns, and non-Hepa which is everything else). It will not make the whole unit HEPA certified for official lead removal tasks, but performance-wise (for all practical purposes related to woodworking) it will be identical to any other HEPA extractor.
It can be used with both plastic liners and standard bags (and you get one bag of each type when you buy a new tool).
With HEPA filter and standard CT36 bags, it will perform identically to CT 36. However, if one adds up the cost of AC feature in CT 36 AC and the upgraded filter, it will be quite a bit more expensive than CT36 alone.
CT 36 AC has a limitation (compared to non-AC extractors) in that it comes with a different hose. It is electrostatic but it has a little different connector. This connector enables one to lock it on the Planex. It works with more or less all tools which are designed to slide the hose connector over the dust port, but will not work with with tools which require the connector to slide into the dust port, such as sanders, without replacing the connector (which is extra $20-$30) or buying a dedicated 27 mm hose (which is in the ballpark of $100).
In my opinion, the only reason to buy an CT 36 AC as the primary dust extractor is because it enables one to add the Planex (drywall sander) at a later time without the need to buy an extra vacuum. You can call it "dust extractor versatility", if you wish. If Planex is not on the list of future purchases, there is no good reason to buy it. It is big, heavy, and bulky. I doubt that boom arm can be attached to it. Large size does not reduce cost of ownership as price of the bags scales with their size. If there is a saving on the bags, it is quite minor. One can use it without fleece bags for sawdust, but using it completely bagless is disgusting (you will be all dirty when dumping the dust out of the vac). You can use it with plastic liners (just as you would when you sand drywall), but the cost of plastic liners is the same as that of fleece bags. Maybe some people see an advantage in larger bag size because if you own the CT36 you do not have to check if the bag is full quite as frequently as with smaller vacuums. I never had this concern, it it takes quite a long time for me to fill even a Midi bag, but maybe I do not generate as much dust as other people. It all depends on the task at hand.
If I could, I would own only the Midis. I think Midi is by far the most useable size, for both work and storage. I would never buy a CT 36, and certainly would not buy CT 36 AC if I had not bought a Planex.
I bought one CT26 for the boom arm. Boom arm requires CT26 because it mounts to the vacuum and utilizes holes pre-drilled in its housing. My second CT26 is used with the Kapex. In retrospective I should've bought a Midi for my Kapex as well. There is a small difference in airflow between Midi and CT26/36 but it is minor and makes little or no practical difference. My CT 36 AC was purchased in a package with Planex. I use Midi in all cases when I cannot do the work on MFT/3 with the boom arm.
I do not have a permanent shop and storage is an important factor for me. If I did not have to pack and store my tools, size of the vacuums would probably matter less. But still, there is no real advantage in going bigger and heavier if one can get the same result with a smaller and lighter tool.
In Europe, Festool offers a smaller autoclean extractor, CT 26 AC, but alas it is NAINA....
