I would stick with the Nexus 4, if you can get one at normal price, since it seems to always be sold out. By the time the Xperia Z comes around in a few months, give or take, the Nexus 4 will probably have had some price drop and you can get a 16gig one for $299, while the Xperia Z is expected to be $599+ or 500+ GBP.
It's a good phone no doubt, but my biggest problem with every Sony android phone I've seen or used or temporarily owned is how bad the viewing angles are. Even head on it looks washed out and there's always a "glow" throughout the image. The above linked video isn't the best quality, but you can clearly see how much better the viewing angles and contrast are on the Nexus 4. It's a true IPS display from LG, whereas the Sony is definitely NOT. For god knows what reason, a big display and TV brand like Sony still can't bring themselves to use a real IPS display on any of their phones. No wonder they are still so far from the front of the pack, last-gen specs and OS aside (xperia s cough). The most prominent thing on a smartphone is the display, and that's how you woo customers casually browsing at the store. That's why the Galaxy S III is selling tens of millions of Galaxy S III phones, because the SuperAMOLED display looks so nice, even though the phone body is pretty terrible IMHO. A company like HTC has the best IPS display in the smartphone market with its SuperLCD, with none of the silly viewing angle and contrast problems that Sony has. HTC doesn't even make displays. Other manufacturers like Motorola and Pantech have the smarts to buy displays from their competitors. Sony needs to get its head out of the sand in this area.
The Xperia Z however has the much better camera. And of course expandable memory to use it. The 1080p samples on youtube look and sound really great, especially since they are in lowlight on a dark showfloor with variable lighting. Leading the pack in fact. Quite a jump from the GS3 and iPhone, and definitely much better than the Nexus 4, although the Nexus isn't that bad.
I think the Nexus 4 has the better shape. The squared off corners on the Xperia Z aren't that great in the pocket, and the slightly larger size will make a difference in the hand and the pocket. The speckled back on the Nexus 4 also looks way better and less smudged. I've watched so many Xperia Z videos, and every time someone says how beautiful the phone is and turns it on its back, I get a bit vomity at all the grease and blobs of fingerprints. The sides of the Xperia and the power button, however, look nicer and more fashionable.