Admittedly, I haven't listened to a lot of the Gardiner set - only a few recordings. Don't get me wrong, Gardiner does a fine job, and I enjoy his recordings of Bach's sacred choral works. It is really just a matter of degrees. To me, as Itullian said, there is a greater spirituality in Suzuki's performances that appeals to me. As for the sound quality, it truly is hard to beat, but Gardiner's recordings would also not disappoint. I wish that my hearing were sensitive enough that I could tell the difference, but suffice it to say that the sound in the Suzuki recordings is crystal clear. Are there huge differences? No. But given the costs you are likely to incur in trying to collect a complete Bach cantata cycle - unless you have the dinero to purchase multiple ones - sometimes it comes down to the small details.
Incidentally, Suzuki's recordings of the B Minor Mass and St. John Passion are also incredible, and rank as my favorite recordings of those works. The St. Matthew Passion is good as well, but just not quite as good as some others out there. I also have the Easter and Ascension oratorios by him, and love them. I have not, however, heard his recording of the Christmas oratorio. I am going to try to remedy that soon, although I really love the Rene Jacobs recording of that work, so that will be tough to follow.
Incidentally, I just found the entire Hanssler Bachakademie complete Bach recordings at my local library, and I plan on listening to Rilling's recordings of the Cantatas - not HIP, in terms of instruments, but certainly using HIP practices, and what looks to be some outstanding soloists.