My personal opinion is that if you are using just the OVF, the 5DSR has better AF (central points at f5.6 and faster but the centre plus helpers is still good at f8), the advantage of FF and is not much outresolved by the 90D because of the absence of the AA-filter. The 90D scores for liveview because it is nearly as good as a mirrorless as the M6 II. I've decided to take the 5DSR rather than the 90D on our next birding holiday because of the FF, and I find that the 100-400mm II on the 5DSR takes the 1.4xTC very well when required, with still good AF.I am satisfied with my 5DsR for birding but I realize that you have your reasons.Still, I think of it as a downgrade. Of course it is strictly my personal opinion and YMMV.
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