Neither - wait for one of the above blocks (Dtek fusion v2, EK supreme, Swiftech GTZ, XSPC edge) to go up on the MM.
Your'll still not get one of them for a £10 beh, Swiftech GT yes, top 4 no way.
The review i just read said the thermalright was between the GT and GTX in temp's so I'd say it's well worth a £10.
It is a restrictive block but then so is your NexXxos XP - high flow system is a must
Edit: reviews conclusion
I was surprised. Thermalright's first attempt at a CPU water block performs well on our test bed, looks great, and has a good bundle included with it, especially the backplate, and colour-coded mounting screws. Little touches like that go a long way. One thing I'd like to see added to the bundle would be an AM2 backplate, but realistically, how many water cooling enthusiasts are running AM2 CPUs right now...
Unfortunately that's not the end of the story. It became clear to me during testing that our test bed represents an UNFAIR advantage for this block. It's an extremely low restriction setup with a very good quality pump running at full speed. For some users, this is representative of what they are using, but most users are either running their D5 (or equivalent) pump at a lower speed for less noise, or are running other blocks in their loop OR BOTH. What that means for the Thermalright XWB-1 is that its performance is no longer chart-topping once you take away these advantages.
At the end of the day, it comes down to the individual user. This block looks great, and provides the best out of the box* performance in a high flow loop. If you're running a low restriction PA120.X radiator with a high flow pump (D5, DDC w/Petra top, or Iwaki) and nothing else in your loop, the XWB-1 is the block to beat.
*it should be noted that the APOGEE GTX needed to be "bowed" with a thicker O-ring in order to beat the XWB-1
If you're running your pump at half speed for less noise, or you have a GPU (particularly with a full-coverage block) or chipset in your loop with your CPU, then you may find the XWB-1 less than ideal.