The 328i basically has no sound stock. Magnaflow sounds like the E90 330i did stock but a little louder and deeper. I think it kind of sounds a bit like the E46 ZHP.
Either one will work and they will sound the same. The core muffler is the same. It's just different piping sizes. If you know what exhaust tips you want to use, that could be the deciding factor as to choosing what outlet size.
AW/Terra | ZSP | ZPP | ZCW | iDrive | CA
Mods: BC BR coilovers | Blacklines | M-tech rear | dual exhaust | M3 steering wheel | BMW SSK | ZHP shift knob | High-kick CF spoiler | NBT Retrofit
M3 front control arms | M3 rear guide rods | Manzo rear toe and camber links | Monroe rear shock mounts | Varrstoen ES1 19x9.5+20 Front, 19x11+25 rear