The first two offer their own plugins so users can log in with it and you can be connected with others users. Apparently it increase visitors and even cuts down on spam. However, I am aware from some users through conversations via Twitter, email and instant messenger, that these two do not always cut down on the spam.
Haloscan, well… it was one of those things I used when I first started blogging – before I got into b2 and eventually WordPress. With Haloscan, you could have your comments hosted with them, but you would have to manually write your blog to your page and then insert the code from Haloscan. It was nifty and I actually liked it until I wanted to have my own, so I coded PixelledComment (I do not offer it for download like I use to. In fact, I took it off in 2006.) Same concept as Haloscan, but I could host all of my comments.
Since then, I have come to WordPress. I have tried Drupal, Joomla, Movable Type, CuteNews, and even GreyMatter. I shudder to think about the last two I mentioned…evil. If you are still using them…do not. In fact, I refuse to link them. 😛
My own opinion is that I really recommend using WordPress and playing with the coding. There are plenty of plugins available just for the commenting system alone. Askimet is not bad, but there are plenty of plugins you can include to prevent bots and spammers.
My suggestion is to try the comment system that WordPress offers if you are comfortable with coding. If not, choose a good system. Other things are to either harden your blog’s security and to make sure to keep your WordPress up-to-date.
What commenting system do you use? Why? If you use WordPress, what plugins do you use for your comments? Do you have any extra suggestions for others who are wondering what they should choose?