That's a hard decision and not easy to advice.
If you get a 6800XT sample that has plenty of headroom for OC then the difference is not that big to many of the 6900XTs.
Here are my experience with 5 RDNA2 cards.
My first XFX 6800XT reference card which I water cooled was an OC monster and was not far from two 6900XTs that I had in my hand (Powercolor 6900XT Red Devil and Sapphire 6900XT Toxic).
Both of these 6900XTs did not OC higher than 2620MHz where as my 6800XT was OCed to 2750MHz.
Therefore these both had about 5% more performance but consumed about 70W more to produce this 5% more performance.
After my XFX reference 6800XT got damaged with ESD, I bought 6800XT Red Devil but this was a disaster card. It did not OC higher than 2400MHz. If I pushed it to more, it would artifact. I returned that back.
After this I went to 6900XT Liquid Devil Ultimate which was then about 10-12% faster than my old XFX 6800XT as it could OC to 2750MHz as well.
Here also the drawback is that my Liquid Devil does not OC at all on memory.
So as a conclusion, it is really hard to advice as all depends on luck.
If my XFX 6800XT would not have got damaged, I would have kept it and did not go for 6900XT Liquid Devil Ultimate as that was really a great card that OCed to 2750MHz with max power setting of 325W on GPU Core where as my Liquid Devil is set to 415W GPU core for 2750MHz.