Don't think you can go wrong either way. I've owned both and currently run Champions. Both have their advantages and with a stock port location there is little difference between them. Earlier Champions had issues with cracking but don't think it is an issue with the newer ones. I currently have 4 seasons on the set on my TSO car without failure. To the best of my knowledge, they have more 8 second passes on them than any other production style head out there we are making a lot of power with them and possibly more than any other stock style head to date.
In a side by side comparisons here are my observations of two heads similarly ported with 1.94 int/1.60 valves in the TA and 1.98/1.60 valves in the GN1:
-TA head is beefy and physically heavier although I hear the latest GN1's have added material.
-Intake ports are pretty equal. TA head had higher peak flow # but GN1's had better mid and low lift #'s. Area under the curve was pretty much the same. TA head does have extra material in the head to raise intake runner and have potentially crazy flow#'s but would require a custom intake or spacers to run.
-Exhaust port is MUCH better on the GN1. This may or may not be a benefit depending on who you talk to. In my combo with the cam and valve train I run it seems to work pretty well.
-Combustion chambers were very similar and the guy who flowed the heads said the spark plug location was better on the GN1 but I am told that my heads didn't have a typical plug location. I hear the new GN1's uses a similar plug location and also have a smaller diameter plug for added material in that area.
-Both sets I've used had the shaft mount T&D rocker setups that are pretty much the same.
-GN1 heads are less expensive than the TA's.
In all I've stuck with the GN1's due to the outstanding customer support and service I've received from them. Tom has been instrumental in getting my car to its current performance level. He is a long time supporter of the Buick community and has been porting turbo Buick V6 heads since the beginning. I would give Tom a call directly for more info on his latest castings.
Hope this helps.
