I've talked to FP residents who did med school clinicals at an opposed institution and they told me that the FP residents didnt get as much attention as at an unopposed place. At the unopposed place, attendings allow you to do anything. Supposedly at opposed places, the internal med /surgery/ etc. residents take the cake and the FP residents many times get left out, because of pre-conceived notions that they dont really need to learn.
I guess either way is fine, but at opposed you would have to struggle more and act more interested in order to get noticed.
To me, I'd just take what I can get into rather than worry what kind it is, but if you actually do have a choice, weigh your options and talk to residents at opposed vs. unopposed and get a general consensus. I've only heard this from residents at an unopposed institution so it might be biased. However, these particular residents felt like they were in heaven with all the personalize attention they were getting, compared to what they have witnessed in opposed programs.