I can assure you that the UE Boom, although really nice for its size, simply cannot match the audio quality or volume levels of the Big Jambox (and every professional review I have read states that). Things would be different if we were talking about the standard, little Jambox but we aren't. You would have to buy two UE Boom speakers in order to obtain the type of sound coming from a single Big Jambox. The UE Boom also heavily distorts once the volume is turned up, mainly with bass-heavy music. I played a few Korn tracks on her UE Boom at near maximum volume and they were heavily distorted. My Jambox Big was playing the song at the same (perceived) volume and that was only at its half-way point, there was still a lot to go before the Big Jambox started distorting. Even then I can turn Live Audio on (it works better at higher volumes) and get stereo separation whereas you need to buy two UE Booms to do that.
The Big Jambox has the UE Boom trounced with every aspect except being water resistant. It should though as it's a $300 speaker going up against something that costs $100 less. They are in two completely different price categories and made for two different purposes. The UE Boom is meant to be an outdoor, really portable speaker that can handle some of the elements while the Big Jambox is more for indoor use where you can really crank things up. I'm not saying that the UE Boom isn't more portable but the Big Jambox doesn't take up much more space than it and the Jambox isn't much heavier. The sound quality, battery life, volume levels, and every other aspect of the Big Jambox makes it a better all-around choice. If portability and shower use are your primary concern, get the UE Boom. However, if you want anything else, get the Jambox Big.