The design of the TDX w/ #5 nozzle and the cascade are not as similar as people believe although similar LOOKING. The #5 nozzle results in a better dt/flow curve, but it's a lot more restrictive so that actual performance in a loop is similar to the #4. What I mean is that the #4 and #5 perform about the same because the #4 yields higher flow rates. The #5 also has the problem of easily getting clogged up with debris in the loop. Combine these two facts and the #4 is a better choice.
The RBX and TDX from Dangerden are really tweaked Whitewater designs and do not share much in common with the "jet in a cup" design of the Cascade and the Storm other than "holes at the inlet".
If you look at the MCW6000, they did decrease bp thickness compared to the 5000 series. They also put a central inlet in that block to get some die area impingement too (just as the DD and Little River blocks do).
Here's a kind of interesting post on my testing of all the Danger Den and Swiftech blocks I have:
http://forums.procooling.com/vbb/showthread.php?t=10872
It's kind of a moot point anyway unless you want to buy 2 waterblocks. The MCW6000 that fits the Socket A is not compatible with AMD64. So you're left with WW or TDX and I'd guess the TDX is easier to use but requires a separate top for AMD64 and the WW is more of a hassle to mount but cheaper.