Honestly, i don't know for sure, so my word might not be the best.
But first off, if you are applying in the spring (2011) neither would even be on your application at all, so it wouldn't matter on that end.
If you mean you are applying in spring 2012 i think it all comes down to WHAT it is you are trying to show to the school you are applying for. Yes on the trip you may do/see/experience more then you would as a CNA, but it would give you no certification that would be in anyway usable toward anything in the USA. The trip i would say would be a great personal experiance, or if you want to show your personality, but if you want to show medical experiance on your application i think you should go the CNA route since that is a standerdized certification that is recognised and usable in the USA. Both probably have very good points to show to the school, and either would be positive, it just comes down to what you want to show them. (also the CNA certification alone i dont think would mean much.....getting it and then working as a CNA for a while is probably what you would need to do for it to really mean much of anything....so if you were just getting the certification, but not working, i would say dont bother, you would be better off going on the trip)
Also CNA is a pathway to start working in hospitals while still in nursing school(especially if you would have concidered working part time anyway doing something in retail or food service or such), and can get your foot in the door for potential nurse possitions once you graduate....many hospitals will consider currant employees for positions before they will consider employees from outside, however if CNA is not required prior to nursing school, many nursing schools do have a point during your nursing program
that once you have passed certain nursing courses you are at that point eligable to take the CNA certification test(so that you CAN work for that experience for a job).
Its all going to come down to how the school looks at it. I think jobs would be looking for experice more then anying. Schools i think look more at the whole person. Thats why they have essays, recomendations and such. They want to find out about your personality, and if you as a person fit what they want as a student to make into a nurse. Also either way they both would give you people you could use as references, nurses that you work with on the trip, or nurses that you work with as a CNA.....either would make GREAT references for school.