what sounds better -- fm adapter or tape adapter?

Definitely the tape adapter...no interference.

What kind of head unit? Both suffer a lot compared to direct RCA jack connection. Of course if you use low bitrate MP3's then you probably wont notice. I want CD quality (not Tape or even worse FM) so I burn at high and large bitrates. Most aftermarket HU's have some sort of RCA input. Easy to find Headphone to RCA adaptors too....

im no audiophile.. its a stock BMW HU ...

I'm just concerned that FM , although more convenient with less wires, might have a tinny radio style sound .... when im using a cd player or an mp3 player i would prefer non tinny sound .....

tapes give me non tinny sound... so a tape adapter will work for sure.. but would an FM adapter be equivalent or "worse" because of the technology/interference?

FM is the worst...it has the least freq range. That's why it sounds like crap compared to CD, or even a tape unit.

If it's an Ipod, there are unit's from Dennison that will tie an Ipod directly into the HU...all control is handled by the HU. Charges too. A bit over $100 IIRC....

new HU