I have been to both. 2012 Olympic Trials in Eugene and World Indoors last year in Poland.
Discounting the fact that World Indoors was in another country and it was my first time overseas, I would definitely pick the Olympic Trials.
Both were great meets and had some exciting races. And World Indoors is a global championship so it has that going for it, but the Olympic Trials is just a different kind of experience. World Indoors is similar to a Diamond League meet. At the Trials, the atmosphere with everyone trying to make the Olympics makes every race very meaningful. The fight for third can be epic rather than an afterthought. All the best runners in the US are there, no skipping this one. No guarantee all the best runners in the world will be at World Indoors. Some will skip, (although I'm sure many top Americans will do it since it's in Portland).
At the Trials everyone is really into it and the meet is getting attention from the general US population since it's "the Olympics" so it's a cool thing to be a part of. Even with it being in the US, I'm not predicting World Indoors will get the same kind of national media attention or hype (although I could be wrong). After the meet each day at the Trials you'll see the elites at the local restaurants and bars and there will be a lot of athletes who didn't make the meet (or who don't make the finals) in town just to watch.
There are some downsides to the Trials though since it's long and drawn out over 9-10 days. So you'll have to take time off of work and spend more money on travel expenses/lodging/tickets. Hotels in Eugene are competitive to get during big events. My advice would be to find a group of people and rent a house together close to the track.
World Indoors is only a 3-4 days, so easier to manage the travel and it's a lot more action packed into a couple key days and you don't have to pay for tickets on as many days. I have no idea about ticket prices for either meet though.
But as the first poster mentioned, it also depends what you're into more. Do you want to see the best in the world or the best in the US? Which are you a bigger fan of? I'm huge into both, but if I'm honest, as a fan of the sport, I think I'm more invested in the races when it's all my favorite American runners going head-to-head to make the Olympics as opposed to many international athletes I'm not as invested in going for a world indoor medal. (If it was an outdoor championship it'd be a different story.)
Lastly, I'll add a plus one for World Indoors would be seeing a US athlete win a global medal on US soil. The atmosphere during the men's 800 in Poland was crazy when (initially, before a DQ) Poland went 2-3:
http://www.letsrun.com/news/2014/03/mohammed-aman-comes-behind-win-15th-straight-800-race-championships-2014-worlds/
Hope that helps.
-Steve