Remember this adage about dive gear redundancy, which holds particularly true for lights:
"Two is one, and one is none."
You should each have TWO lights on a night dive. This way, if a light fails you would each have a backup on which you could COMPLETE the dive. (Note the difference between the word COMPLETE and the word END.)
With only ONE light each, if one of you has a light failure not only will you need to END the dive on one light between the two of you, but it may get scary pretty quickly. (Do you want your wife to end a dive, ascending without a light, not being able to check her computer, etc?)
At a bare minimum, I would suggest that you guys dive with at least THREE lights between you on a night dive. This way, if one of you happens to have a light failure there's at least ONE backup available with which to END the dive. (Again, END vs COMPLETE.)
Of course the absolute right answer is that you should each have TWO lights, so that if one of you has a light failure, you can switch to your backup light and COMPLETE the dive (Do the whole dive you planned, instead of ENDING the dive at the point of light failure) because you will still have ONE MORE backup available to the two of you.
You and your wife should get the SAME primary light. Don't cheap out on trying to use a "primary" light for one of you and a "backup" light for the other. The one with the backup light will really hate the fact that their light is less bright/effective than the other. It will literally make it hard for the backup user to see as their light will be overwhelmed by the other. Also, the one with the primary light will have a harder time seeing the backup light - remember a light is also your primary signalling device on a night dive. If your light is brighter than your wife's (or vice-versa) you will have a much harder time seeing her beam if she is trying to signal you. (I know you may say "but we stay right together all the time and always look at the same things so we don't really need to worry about this..." but if something goes wrong, the extra $10 or so will be a non-issue.)
Also note as llqwyd points out above, a light is very usefull on day dives as well. A good handheld light is great to peek under a ledge or behind coral or whatever - you'll see plenty of stuff you will have missed otherwise!
As to which lights to get, UK and Princeton Tec are both great. You cant go wrong with comparable products from either. Comparing the Princeton Tec Torrent to the UK SL4 (LED or Xenon for either) I prefer the size/shape of the Princeton Tec, as its traditional "round" flashlight shape is more comfortable to hold than the UK, which is sort of flat/wide. I also find the switch on the Princeton Tec to be less prone to being accidentaly switched on or off - either during a dive or even in your gear bag.
I also have an affinity for Princeton Tec based on customer service I've personally received. I had a Princeton Tec Surge that died after about 100 or so dives. Opened it up and there was a bit of corrosion on one of the contacts. Likely "user error" on my part in terms of poor maintenance. Took it back to my LDS. They told me PT would repair or replace no problem, I just needed to send it back for them to take a look at. They called PT who told my LDS to simply replace it on the spot out of the shop's inventory and they would send the shop a replacement. No wait, no cost or hassle associated with shipping, I just walked out with a brand new light. In fact the light had been upgraded to the newer version (the Torrent) since I bought mine, so I got the newer one. And since they come with batteries I got a fresh set with the new light as well. You might say that the light crapping out on me was a pretty good thing.
The Princeton Tec Torrent LED is a great light at $59:
Princeton Tec Torrent LED at www.DiveSeekers.com
If cost is a factor in your decision, save on the LED and get the Xenon version, available for $39:
Princeton Tec Torrent Xenon at www.DiveSeekers.com
Either way, get a Torrent for each of you, and then get at least ONE Princeton Tec Tec 40 Xenon as a backup for $17:
Princeton Tec Tec-40 at www.DiveSeekers.com
I can highly recommend
www.DiveSeekers.com as an authorized Princeton Tec dealer. If you email or call them (888-728-2247) I'm sure they'll be happy to discuss your needs further to make sure you get exactly what you need, and perhaps even put a "package" price together for you. Ask for Wayne or Jay - the owners - and tell them I sent you. They'll treat you right!