Peace (not sure whether that's a name or a salutation, but welcome)? ?

Two big differences.
The waist width on the RX8 is 66, the waist on the TigerShark is 73, and narrower waisted skis are much easier to learn on and to get up on edge.? Peter has a couple of articles on the main site that explain the physics of this, but the farther the center of the foot is from the edge of the ski the more effort is required to tip the ski (sidecut can offset this to some degree).
The RX8 is probably available for 1/2 the price of the Tiger Shark because it has been around for several years; but it was a VERY popular and versatile ski for the past several years.? In some minds almost a classic.
I don't know any that have skied the Tiger Shark, but the reviews here are certainly very favorable, and all indications are that the Power Switch does make a difference.? However, you may also want to look at the 2008 Reviews section and check out the "Skis of the Year" in some of the other categories such as "Skill Development" and "Speed Carving".? I will admit to a certain bias, I have always found Volkl skis stiffer than really interested me, so keep my bias in mind with this.
I would certainly take Peter's comment about "this ski must be tested by the skier" to heart.
PS. I moved your post up here for fear it might not get noticed.? Due to problems with spammers, the forum software prohibits "newbies" from creating new Threads under the first heading.? Welcome.
Jim