If you're looking for the 20 series, i suggest suggest you get GGH20 if you want to use it for long distance travel. Otherwise, go for ANH20 if you do frequent city driving.

I have both GGH20 and ACR30 (similar engine as ANH20) and i find that driving the 3.5L is more fuel efficient compared to the 2.4L on long distance travel, plus the engine is not as stressful. The RPM is usually at 2.5k @ 160kmh and below 2k at 120kmh.

One thing for sure, resale value for GGH20 is much lower compared to ANH20. Regular maintenance cost is a bit higher for GGH20 :
1) 6.2L of engine oil for GGH20 vs 4.3L for ANH20.
2) 6 pcs spark plug for GGH20 vs 4 pcs spark plug for ANH20
3) Labor cost for spark plug is definitely higher for GGH20. It take hours to replace the spark plug due to the plugs location

However, there are certain things that cost lower for GGH20 such as ATF oil and oil filter.

You are right about the 6AT.. I have friends already experienced some issue with the 6AT, but so far I have not experience any problem since 2013 *touch wood*. This is probably because i had installed original Toyota ATF cooler in my car which cost a bomb, unlike some of them who wither use 3rd party cooler such as Hayden or none at all.

From what i heard, typically rough/lag shifting from one gear to another usually during at below 50kmh.

Most JDM car do not come with an ATF cooler, which is quite sad actually. The one that i installed was meant for 3.5L Alphard/Estima taxi in Japan. However, I'm not so sure whether those sold by UMW comes with ATF, but i think they should.