i have a vest, acapella, flutter, and a pep. my personal preference is the vest, i think it works the best for me and i always travel with it except for two trips out of the country when i wasn't sure about the wattage exchange etc with the outlets. didn't want to risk blowing myself up or something <img src="i/expressions/face-icon-small-wink.gif" border="0"> point being i took my acapella and it worked just fine. the difference between the flutter and the acapella is that the flutter has that metal ball in it that you blow against for resistance and the acapella does the same thing but uses valves or something-it doesn't come apart like the flutter....i like it better because of that....i would say another downside is that you can't do hypertonic saline while you do it, like you would with the vest but if your son doesn't use that anyway then it won't matter!