I carried 6 for a short while but decided I wanted a stockier blade so I bought a 5. Its just that-Stocky. I agree that the 5 might be overkill but something like the 3 might be "under-kill"
The ESEE-5 is like a 8lb sledgehammer. Its big heavy and can do really big stuff. That said at times the job may call for a 8oz ball peen hammer but with a little finesse the sledge will do the job.
The ESEE-6 is like a titanium framing hammer, to light to do really big stuff but too long to do really fine stuff. Even still its a great "Middle-of-the-road" knife
The ESEE-3 is that ball peen hammer, built for small jobs and odd jobs but with a lot of work it can do some larger jobs too
These metaphors might not make any sense to a person who isn't really familiar wit hammers but its the best metaphor I could think up.
Also, I love the ESEE-5, people will tell you that its too thick to do anything but last weekend I was sitting around the campfire making little 4-figure type things for my little cousins to play with, they loved them and I didn't think it was too hard. Even with that thick blade.
On the same trip I did a massive amount of batoning which the 5 shrugged off as if I was splitting pencils.
Hope my sleep deprived ADD-style writing made a little sense and good luck deciding. My 2 cents is to go with the 6, I personally prefer the 5 but it's certainly not for everyone. The 6 however is a very easy-going kind of knife, hard to go wrong.
P.S. Don't let people tell you the 6 iss too long for anything. It's a long blade but its thin and really handle heavy.