They both borrowed so many ideas from each other that the architectures are nearly identical these days. Neither modern ARM or modern x86 architectures deserve to be called RISC or CISC.
Still, there are a few areas where the A64 architecture is theoretically "better" than x64, due to the lack of legacy.
The first is instruction decoding, x86 had to deal with a whole bunch of weird instruction length modifiers, prefix bytes, weird MOD RM encodings and years of extensions. To decode the 4 or 5 instructions per cycle that modern out of order microarchitectures demand, Intel CPUs have to attempt decoding instructions at every single byte offset in a 16 byte buffer, throwing away unwanted decodings. That's got to waste transistor and power budgets. In comparison, when an ARM CPU is in A64 mode, all instructions are the same length, making decoding multiple instructions per cycle trivial.
The second area is memory consistency guarantees. x64 has relatively strong guarantees, allowing simpler assembly code but at the cost of more complex memory subsystem between cores. A64 has much weaker ordering guarantees, which saves on hardware complexity, but required the programmer (and/or compiler) to insert memory fences whenever stricter ordering is required.
This is all theoretical, I have no idea what difference this makes at the practical level.