Phnom Penh or Vientiane?
 

On a short trip this summer, I have time for one but not both. Any opinions?

What is the rest of your itinerary?

I personally like Vietiane better. It just feels smaller and friendlier. It's also much cheaper to get great food and drink!! The Mekong River also holds more mystique for me there than the Tonle Sap confluence does. Either one is a fun and great place to visit.

There's probably more to do in Phnom Penh both in terms of nightlife and tourist attractions.

http://www.travelsinasia.com/Cambodia/PhnomPenh.htm

Vientiane is nice enough but a bit sleepy, OK if you just want to relax in between places.

KimJapan, I'm pretty certain I'll also be visiting LP and SR, my second visit to both places (and in and out of Bangkok). Either work pretty well from a logistical standpoint.

Definitely Phnom Penh. There is far more to see and do than in Vientienne and there are some really great markets (although some might consider the visits to the Killing Fields and Tuol Sleng a bit disturbing. Also there the food is much better than in Vientienne. Personally, if you are only on a short trip, I would check out the logistics of travelling betweenthe four places you mention as some could be a lot easier than others.