The A6M3 is significantly better than the A6M2, while still being slower than the A6M5.
That said, if I had to pick between it and the Bf109G-14, I'd take the Bf109G-14.
The Bf109G-14 would go in the gap of more than a year and a half that we don't have anything for the Bf109s. The Bf109G-6 is early 1943 and the particular Bf109G-10 we have, really a K-4, is October, 1944. We need a Bf109G-14 for early 1944.
The Spitfire F.Mk IX we have is from mid-1942, the Spitfire Mk XIV didn't see combat until March, 1944 and wasn't available in any numbers until the 4th quarter, 1944. A Spitfire LF.Mk IX or Spitfire Mk VIII would fill in that gaping hole. Comparitively, the P-47D-11 does fine as a place holder for the P-47C or P-47D-5 and the Fw190A-5 is OK as a placeholder for earlier Fw190s.
The two biggest holes in the planesets of types we already have are the Bf109G-14 and Merlin 66 powered Spitfire.