A20 is indeed only 2 cores while H3 is 4 cores, so H3 is more powerfull on the paper. Sounds like a downgrade but actually A20 is more expensive than H3!
For emulation raw power was not the limitation on RetroStone1, it was rather driver support and video acceleration the main issues.
With A20 we have similar emulation, but A20 has much more advantages.
First it has the digital LCD interface + HDMI out.
Secondly it has a battery management power chip associated. Which makes system more stable.
Also A20 has a lower power consumption, so a better battery life.
And last A20 has a good support for linux mainline. Meaning better drivers and better performances.
Overall for our game console application A20 is currently the best candidate.