I had problems with stanza but I think I was trying to add to many books in one go for it to handle but still I ditched it afterwards, Ibooks hasn't given me any issues and it's convenient and also has the store built in but I've been doing most of my reading on i2reader. I wouldn't say i2 reader is really any better than the other two I just have grown accustomed to it.
I love Stanza, but I didn't try any others. I particularly appreciate the ability to change the font size, go reverse colors (light grey on black in my case - great both for reading outside and in the darkness without causing eyestrain), option to lock orientation and ability to load a great variety of text formats into it. In your place I'd just try it - IIRC it is still free, isn't it? As is iBooks, I think.
Does iBooks store your novels in your device, like songs for iTunes? I use the Kindle app and I have a ton of novels that are stored in a cloud, I guess. I dont see any space on my iPhone being taken up from my Kindle app.
