For me, ideally I wanted a 980, but not for any particularly good reasons. I just remember when they were new, and I always thought that the 980 was cool... in part because I like overpowered airplanes!
When I started looking, I assumed I'd get an 840 with dash 10s, because they are more common. And, functionally, there isn't much difference. I slightly prefer the simpler operation of the 980 with the single red line computers, but that really isn't a big deal.
I definitely wanted a Jetprop over a 690 because of the lower maintenance requirements, and the many small improvements. The redesigned wing in the Jetprops is a big deal.
As for a 900/1000, I did give that some thought. I have a lot of experience with 1000s, and in fact before I bought my 980 I was renting a 1000. However, after giving it some thought and consulting with my wife, we decided that we preferred the interior configuration of the 840/980 to that of the 900/1000. The big window and aft couch of the 840/980 are just too nice. And, we don't generally carry a bunch of people, which is where the 1000 would be better.
I also have to admit that a 1000 would have been more upfront money than I wanted to spend on an airplane, but that wasn't really the biggest factor. If a 1000 had been what we really wanted, I could have found a way, or got a 900 (what my friend Norm tried to talk me into), or something like that.
As for why I ended up with a 980 rather than an 840, mine was the only 980 on the market at the time, and it really was the best fit for me, and, in my opinion, the best one on the market (840 or 980). Bob Mays told me it was a special airplane, with it's very low time, two owners since new, etc. and while he may have been talking it up to close the deal, looking back I think he was absolutely right.
Sorry to boast, but I think I lucked out-- it's one of the best Commanders I've ever been involved with-- and I've been around a lot of them.
