I use all three options, plus others, but I'm voting for tablet because I've run out of shelf space and my Kindle is a lot more convenient than my laptop (it will fit into my pocket, for example). Also, I'm using DVDs to store mob or other readable format files - that consumes much less space than paper. Of course the kindle and the laptop can both display pdf as well as mob format, and conversion from other formats to one of those is usually extremely easy. I've found that sometimes the laptop is useful for ebooks with diagrams in them - diagrams are sometimes badly handled on the Kindle.

The scarcity of shelf space means that I'm now buying a lot more ebooks than paper ones. But it will be a verylong time before my ebook collection has as many books as my paper collection - the paper collection had a very long head start.