I used an iPod Touch for this before I got an iPhone. I also have a 1st-gen iPad, but I rarely use it now that I carry a Macbook Air on location.
My wife just got an iPad Mini, and I am surprised at how much I like it. I think it would do a fine job as a digital portfolio, and fits in more places than the larger version.
+1 on the mini. I got one for a location device with cellular and I love the thing. I have a full sized iPad3 but its not seeing much use these days. The mini is easy to carry and use and it makes a great display device for showing images. Plus everyone wants touch and try it