Both breast implant companies make a great implant and there is little to chose between them.  However, I believe that Sientra implants have a couple of advantages.  The first is Sientra implants feel almost like your own tissue at 3 to 6 months and women who have Sientra breast implants rave about how natural they feel.  The second is that Sientra’s gummy bear implants come in many shapes and the classic tear drop shape can provide your breast with not only volume but improved shape as well.  And finally, the warranty that Sientra offers is far superior.  Their implant warrantee covers capsule contracture and 20 year coverage for failure that includes funds for surgery, without having to pay extra.
I have written a link to a two part article about why Sientra implants are so good that  you may enjoy.
As you do your research, make sure you look for surgeons who perform 24 hour rapid recovery breast augmentation as this will help you return to the gym much faster if you are athletic.  It also reduces your overall complication rate.