The doubling of the ROPS is a big boost... And one bottleneck of GCN gone. - Reducing wavefront size and increasing SIMD size so they match is another big Pro' as well and another GCN bottleneck gone.
It's going to result in a substantial uptick in performance per teraflop either way... I would not be surprised to see a 50% performance uplift over the RX 590."
RDNA has a SIMD32 and a SIMD64 mode which corresponds with their dual scalar units and is somewhat similar in concept to what Intel has been doing a long time with variable SIMD widths on their GPUs ... (Intel has SIMD32/SIMD16/SIMD8/SIMD4x2 modes)
As for primitive shaders, I'd be interested in seeing an API exposed for it to see what it's programming model is actually like as an interesting comparison to Turing's mesh shaders. "Primitive shaders" are still a massive mystery since we still don't if it's truly equivalent to Turing's mesh shaders or something completely different ...