The biggest differences, IMO, are the obvious ones:
- The Clara 2E and Clara BW both offer 16GB of storage, compared with 8GB on the Clara HD. Are you finding yourself running out of storage?
- Both of the newer models have better-positioned and easier to use power switches.
I don't find the screen of my Clara BW to be noticeably better than that of the Clara HD.
If you're happy with the Clara HD, don't change it.
Reading PDFs isn't noticeably better on the Clara BW: it's still pretty awful. This is one of the few things for which I'd reckon a larger screen would be beneficial. But, of course, that comes with penalties in terms of price, weight and portability. Personally, I'm not convinced that the 7" Page or even the 8" Sage are really big enough for this.