I think only you can tell whether the 306 is worth the extra money compared to the 302. I've never regretted getting the 3" x 8" 306 UF benchstone, myself.
I use it in an adjustable Shapton stoneholder, and sometimes just as a handheld finishing stone. I got it on the recommendation of Michael Christy and Shawn Houston.
Mine needed a little finishing work, as it had kind of sharp edges and noticeable machining striations on some parts which you could feel if you ran your fingertips over the surface. After a bit of lapping in, and smoothing off the edges with SiC sandpaper, it's been a great last stone to use, both in my Chosera waterstone progression, and my Atoma/Venev diamond stone progression.
It works very nicely for large kitchen knives and chisels and plane irons too, which I believe is one of its intended purposes.
I really like it, I think it's a wonderful stone, especially on knives to be used for food, or which are used for regular push cutting type tasks.