Jeff:
Jesse and Wino's advice is really very good, as usual.
The CT 26 and CT 36 are essentially the same machine except for the capacity. There are 10 liters of potential bag capacity difference. If you are using either of them with only a TS 55, I suspect you will want to change the bag before that difference really matters because of the density of what they contain.
Of course, that also depends on whether or not you need to move the CT. If you can roll it around wherever it needs to go, you can fill the bag fuller with more dense materials like fine sawdust. A CT 36 alone is quite easily manageable to me. A CT 36 with 8-9 gallons of fine sawdust is what, 50-60 pounds heavier?
I may be exaggerating, but I don't think by much...
I've carried my female customers' CT 22 from their truck to my "showroom" (can you spell chivalry?) with a chock-full bag. It was not pleasant. The bag turned out to be so full that the CT was plugged. It was like a brick and it took some persistent, gentle persuasion to remove from the housing.
I think I know the history of that particular bag and it was largely 22 liters of TS 55 in MDF and maple, plus a lot of sanding of the same. It also included some exotic species that were cut and sanded to about 180. It got really compacted, to the point that the CT 22 didn't want to suck any more. The whole thing was pretty darn heavy. Add another 2/3 to the weight (22/36) and it would be less pleasant.
I use this bag as an example because it's the worst that I've run across. It took us a little while comparing it to my similar unit to determine that the over-full bag was actually the problem. It didn't look full but was pretty darn full and turned out to be a real "brick".
The larger capacity is very helpful in situations that require larger capacity but can also hamper mobility if larger capacity isn't really needed.
Tom