Oh dear tough question both colors are very diffrent, if your looking for something versatile and understated I think I would go with marine if your looking for something unusual that pops I would def say BI. I saw the BI city IRL and it was just amazing. The color pops without being flashy !
someone posted a pic of marine on here and i guess it's on naff's site now so you can check it out there...it is a really pretty color! i didn't expect to like it as i thought it'd be more like a black but it's a really pretty dark blue.
but yes BI and marine are totally different colors, so hard to say which ... but i do have a BI Day, i just LOVE the BI color.
Like others have said, the colors are so different it's hard to say. I agree that BI is a nice pop of color without being too bright at all. Marine looks like it might work like a black bag, but also looks gorgeous. Do you know what style you want and how you will be using it?