I voted LED, but the poll is wrong on several counts.
- There are no LED displays that are portable. Or even viewable for closer than ~50-100ft. The poll should be about LCD displays (LCD displays lit by LEDs as backlights are still LCDs) edit: I guess technically AMOLED could be considered a form of LED, but it's relatively rare (only a handful of phones use it, and some phones that did use it stopped doing so due to production shortages) and it's distinct from a traditional LED (those can't be made small enough to be usable as a pixel grid for close-up viewing).
- There's no variance for display size and pixel density.
When it comes to LCDs, size and pixel density are what really matter. For best reading comfort you need something with a pixel density of 150ppi, ideally higher (over 200ppi). That rules out pretty much every PC display in existence as well as pretty much every tablet over 7". It's not until you get down to phone size devices (3.5-4.5" devices with 480x800) that you break the 200ppi barrier (the non-retina iphones are acceptable with 3.5" screens at 320x480 and 165ppi, but just barely). This is why reading on my 4.3" 480x800 phone is as comfortable to me as reading on my Nook STR, but reading on the girlfriend's iPad (132ppi) is painful for more than brief usage like web browsing.
With proper lighting control e-ink will always be better. However I rarely read under ideal lighting situations and the portability of a phone means I always have the device on me and thus LCD is where I do 90% or more of my reading.
For those that think backlights give them headaches, try a high-ppi device in night mode (white text on black background). I bet you'll find that's much more comfortable than reading black text on a white background on your 15.6" 1366x768 (100ppi) laptop.