I've been lurking these forums for quite some time now, and two days ago I pulled the trigger on a set of Beyer DT880 600 ohms. I'm deciding between the LD MKIII and MKIV as well (the IV SE is definitely out of my price range). I was actually hoping you would get a response on this thread, as I was asking this exact same question myself. I actually checked the thread a few times, but figured since it made its way to page two that it was not going to get any responses.
I've contacted David of Little-Dot, asking for his opinion of the very same question. Here is the response I got:
"I do indeed recommend the Little Dot MK III to most users due to it's price/performance value, however if you would prefer to have better sound right from the get-go and not worry about tube rolling, the upgrade to the Little Dot MK IV is definitely something to consider. You would then not have to worry about upgrading as well. [...] If you have any other questions or requests, please do not hesitate to ask!"
I've read just about every thread I can come across, and there is no clear winner, though not many have listened to both so far as I can tell. Most people are happy with either, so obviously they will both sound good... but its hard to tell if the $100 price increase is worth it. Knowing me, I will probably tend to go for the IV just because I'd rather have more than less, and I really don't want to have to upgrade down the road to get just a bit more performance.