I haven't used either, but based on what I've seen with the Tango I'd think that the Share, when used in a harness, would transfer some of the pull to the harness whereas the Feeldoe would concentrate the force from the pull (the pull is the force from pulling the dildo out - does this make sense?) inside the vagina/on the G-spot. You'll still get some stimulation from both of them, the question would be how much do you want.
The Feeldoe also has a vibrator, whereas the Share doesn't. It's just a little silver bullet, and I don't know how stimulating it is, but if you like that sort of thing...
As a getter, the biggest thing I'd notice is that the Share is softer silicone, but has more dynamic range (bump-thin-thick) as it is thrust in and out, whereas the Feeldoe is (via rumors) a bit stiffer and roughly the same size all the way down. Both have a noticable curve, so they'd both hit whatever "spots" were present.