Just to clarify, there is a performance increase with the latest cards like the Delkin Black. The increase is modest and does not affect frame rate which is already high. You will potentially have faster download speeds from the card. You might see a decrease in maximum burst length from 200 frames today to a lesser amount if you choose a slower card. If you go with a slower or low performance CFExpress card like ProGrade Gold 128 GB cards, it's actually slower than a good XQD card. So it varies and depends on your choice of card. That's why I'd suggest a Delkin Black, Delkin Power, or ProGrade Cobalt.
CFExpress cards essentially future proof you a bit. Some of these cards are much faster on the latest cameras. There is a clear move to CFExpress and XQD is not going to be provided by manufacturers at some point in the future.
I'd also look for upcoming sales related to Black Friday / Cyber Monday. Memory cards are often discounted. I saw a deal yesterday from Roberts Camera on a 256GB Delkin Power card for $199. You may not need a card that large, but on a cost per GB basis it was a great deal.