Besides the fact that diamond tweeters are $700 (65-13-9-350-327) more expensive than the -329, I can only offer opinion.
I considered using the -327 tweeters in my retrofit, so I asked the crossover designer I was using as he spent a career designing high end speakers..he offered this:
"The main advantage they [diamond tweeters] have is that the membrane works as a piston higher in frequency allowing greater clarity but other than that, they have a rolled off top end and sound harsher than metal. The metal domes can be made thinner and easier to dampen the ringing, also the metal domes weigh less and had more output at the top..........there is much more to be said, but won’t go into it too much here. I designed many hi-fi tweeters. Personally, I would go with the metal dome over the diamond dome as the diamond will have a lack of air due to its fall off above 10khz. Since you are listening off axis mostly in a car, you want a boost above 10khz, not a roll-off"
I can't add much to this assessment. I'm using the same tweeter now (65-13-2-622-773) w/o the filter and I'm happy with the improved clarity. I find them well rounded and not piercing, but everybody's interpretation of sound is subjective. Over a year ago you could source used diamond tweeters for about $330/ea on ebay, but I don't see any on there today.
UPDATE: I almost forgot, I did roll off the top end when I tuned the system with RTA and microphone at the driver's headrest.
My graph is in this link plus a lot of discussion on these speakers:
Best Audio Upgrade for the X5:
https://www.xbimmers.com/forums/show...php?t=1774331]
I hope this helps.