both soundlink and BIG jambox cannot reach the sound quality of the sounddock. the sounddock plays much louder (i would say twice as loud) and has a more balanced sound with deep powerful bass, that both soundlink and BIG jambox cannot reach.
as both soundlink and sounddock are made by bose, they indeed both sound pretty similar as well. but because of the soundlink's smaller size and different drivers, bass is tuned differently. the soundlink has quite a powerful bass, some even say overpowering, but it won't reach that deep like on the sounddock.
funny thing is, that outside the soundlink tends to sound more powerful than the sounddock because of the stronger bass emphasis. so for outside use the soundlink really shines if you prefer powerful sound. the treble is the weakest point of the soundlink and due to the strong bass it might even sound a bit recessed, especially if listened in small rooms and put in a corner etc.
i compared the BIG jambox and the soundlink side by side lately and i was really disappointed by the sound of the BIG Jambox. despite being quite a bit larger than the soundlink, the sound was not that powerful, it could play louder than the soundlink but already distorted heavily. the soundlink sounded more expensive to me, more refined. the jambox sounded a bit brighter, but bass just didn't have the power of the soundlink.
if you want a similar sound to your sounddock, i would go for the soundlink as it resembles this sound characteristics much more than the BIG jambox. besides the soundlink is perfectly portable, as it is not larger than a book. try carrying the BIG Jambox around, that has the form of a brick!
if i were you i would still try listening to both and make your own decision. i liked the soundlink more soundwise, as did most of my friends, but there are enough people around who prefer the BIG jambox.
btw. on the soundlink you can apply a trick in order to put the bass in balance with the rest a bit better. just crank the volume on the soundlink to the maximum and control the volume level on your player instead. the soundlink reduces bass amount at its highest volume-settings, so it might sound better like this especially if you play music at lower levels.
