Differences:
First, a pic of the expansion tank mouth. This is an old tank of mine that I replaced because it had a slight leak at a seam. The white stuff is wax, to see points of contact between cap and tank.
http://i239.photobucket.com/albums/f...psqkjtwbmb.jpg

Stock cap has a rubber seal that engages the step in the tank mouth.
http://i239.photobucket.com/albums/f...ps8xbx0ftg.jpg

E93 cap doesnt have this seal, it has an O-ring that engages the inner wall of the tank mouth. However, the measurements are different enough that the O-ring doesnt engage the tank mouth. Wax didnt smear in that area when I tried the E93 cap.
http://i239.photobucket.com/albums/f...psckkajkbk.jpg

Also, if you will look at the tank mouth pic again you will see 2 bands where the cap valve O-rings engage the tank body when the cap is screwed in. Problem is the E93 cap valve sits higher and screws in less than the E53 cap, so the valve will not be in the same position.

http://i239.photobucket.com/albums/f...pscaldihsq.jpg

TL,DR: Cap threads are the same and cap valve is same diameter but E93 valve lacks upper seal and sits in a different place in the tank compared to an E53 cap.

That is one big ass ugly cap, E30 cap ftw!

If you want another option check german auto solutions I have the billet caps on both my 540 and e53 they work and look great

The GMS caps are nice! For this particular part under the hood I personally am not wanting to spend more than necessary though.

RB, it actually looked decent when it was on. Filled up the space and was flush in the area, did not stick out like a sore thumb like i had imagined. I wouldnt have minded using it at all if the valve fitted properly.