I have backed project on both, though I tend to prefer IGG. Not for any particular reason, except maybe the UI.
I don't feel there is much difference for a backer, and the way you interact with projects and use the sites are pretty much the same. They also have some of the same issues, though I have found a bit more on IGG, since I use that site more.
Luckily I have not had that many bad experiences with projects not delivering, but I do feel, that I would get nowhere with either if there is a problem. They tend not to react very fast, even when campaigns are flagged for fraud/scam.
Example: On IGG, I backed something called Kratos (a wireless harddisk for filesharing). No products were ever dlivered and there were no updates during or after the project ended. For a long time the perks were still active, and IGG would not disable them as that was the responsibility of the owner of the project.
I have no assumption that Kickstarter would be any different.
But I just try to use some common sense when I select my projects, and so far everyone (except Kratos) have delivered, or seem likely to deliver my perks.
I also see quite a few projects start on Kickstarter and after the campaign ends, start one on IGG to get more backers. So whether you use one or the other, seems not to matter so much to me.