In earlier years I would have described the more linear styles as "hard", the flowing circular styles as soft. But I have come to believe that hard and soft aikido has nothing to do with any particular style, more with how I achieve the technique.
I try to practice soft myself. What I mean by that is that when uke attacks me, he wants me, I try not to be there. I try to move and create a hole or void where I was standing and guide uke into that hole. Once he's falling, technique can be applied simply and effortlessly, sometimes without pain (probably not the likes of nikkyo though
If I don't move enough, or if I try to do a technique too soon, or uke stops, whatever, I will become the conflict for uke. In order to make the technique work at that point, I have to apply more effort, energy and strength. This is what makes the technique hard, at least to me.
I don't think that hard or soft is the issue, for me, I want my Aikido to be powerful, this can be achieved many ways, linear or circular, as long as my timing and movement are good.
I wish I could better explain my thoughts in writing.