I am most comfortable calling it a disease. I have a myriad of illnesses on my plate - I don't know why I would call depression with suicidal tendencies an illness, but diabetes that can be fatal if left untreated is less severe and 'just a condition'. Both have equal impact on life and daily management, so I will call them by the same name.
I face many issues with people not taking diabetes, or illness in general, seriously enough. My depression is responded to with "well just cheer up and do something fun", my anxiety is responded to with "well just relax and do something fun" and my diabetes has been responded to with "well just eat less sugar and you'll be fine". This is incredibly harmful even if most people don't realize and don't mean it poorly.
While well managed diabetes isn't an issue, even well managed diabetes can go horribly wrong. There are no guarantees, and the last thing I need in my life is to have institutions and healthcare professionals dismiss the impact these illnesses have on me. If they don't see it as 'bad enough', I will not get the help or support I need. I had to jump through so many loopholes to get my disability simply because I was too well spoken, articulate and smart. I could describe my issues clearly enough that they thought I was faking it. The same seems to happen when I refer to diabetes as a condition instead of a disease. If I can talk about it in such a nice way, surely it's not that bad, right?
This leads to quite a lot of misunderstandings and people not realizing that I need some extra space, time and accommodations, extra awareness from the people around me and preparations that other people don't have to take. If I don't refer to diabetes as a disability, I may not be granted extra privileges like taking food and water with me to places where it's not allowed, even though I need to have both on me due to diabetes. If I do not call diabetes a disease, people respond to things like specific food or drink requests as 'you can cheat on your diet this one time'.
For diabetics themselves, I totally understand why condition is more preferable. It does sound awful to call it a disease when on most days we're doing just fine and, while it's annoying, it's not as dramatic as 'disease' makes it sound. But at the end of the day it is a life or death matter to me, and I wish others would treat it more seriously. The only way people seem to take it more seriously is by calling it a disease.
(Gosh sorry for the long winded rant there. I just want to make sure there are no misunderstandings on my points!)