I could be wrong but my understanding is that format doesn't really matter as far as the engine is concerned (as long as you are using a lossless format), meaning that whatever format you bring in it's going to convert it to the data/format the engine reads. So basically it comes down to whether you want to use compression to save hard drive space, though this might increase import times as unreal has to uncompress it first, or leave it uncompressed with faster import times but much greater disk space.