I was under the impression that 6.9s have the W123 part, with only the early versions (like mine) having the extra-long inner end. I would suggest that OEM brands will last a lot longer than aftermarket.
I replaced mine about 6 months ago with the cheap aftermarket Karlyn brand and the boots have already cracked. The car has only been driven around the block a few times. In contrast, the original TRW drag link still has perfect boots and they are right next to each other. Lemfeorder, TRW and Febi-Bilstein are the ones to go for.
You can purchase the 6.9 version OEM ends by themselves from the usual online parts vendors.
Edit: Even if you have the early version with the long inner end, I suspect that the outer ends are indeed the same as other versions, unless they are a different diameter. I haven't checked that.
Without double checking, I think the whole inner/outer assembly is 6.9 exclusive. I think you can interchange with other, cheaper units, but they would not be as strong and also Ithink that a lot of "screwing out" adjustment is required. It's not my preferred option. Regards Styria