I admittedly don't use the Swami as much as I'd like, but I do have enough experience with it to distinctly favor the Boon-style writer over the (standard?) nail writer. Personally, in my hands, it feels more comfortable and allows more freedom of movement for the thumb. In addition to that, it's more angle-proof when worn correctly. I find that I can write certain words and messages more effortlessly than the nail-writer and I've a friend who agrees with this line of thinking.
Honestly-- I would recommend just getting both styles and seeing what works for you. The Boon and nail writer would probably only cost you a total of $10-20 at most. If you decide that you enjoy using the Boon-style Swami, I think you should invest in the Super Sharpie. It was a utility device that was released in the mid-90s that was essentially a standard sharpie pen with the added bonus of a Boon attachment. The prop looked absolutely harmless... so much so, that I absent-mindedly lost mine years ago.
Hope this helps out.
RS.