Hmm, for me its kinda wierd. I would take the natural IF she was young / no kids.. To me, implants are to restore the natural beauty of the breast, not "just make it bigger" Age and childbirth take its tolls, and a way to reverse that is with implants.
I fully plan on getting my wife implants. Sure, she's only 23. But we've had 3 kids. That takes its toll man. Yeah, she still looks very good (You couldnt tell she has had ANY kids, let alone 3) but there are a few things me and her would like out of the breast. She wants them a bit perkier, and I would agree. She may want a bit bigger, but I think hers and Ka-pow right on.
So, all things being equal, I think I would take the natural IF she was young / no kids. Because as I said, implants are to reverse the wear and tear of life, not to enhance an already very nice set.