I don't actually think James Franco's impression / impersonation is particularly accurate, he exaggerates a lot of it and I'm not really sure why. When people say he "IS" Tommy, I just can't see it.
That being said, whoever he becomes, he does become someone. You completely forget it's James Franco to the point where Tommy and Greg are speaking and you always see them as the characters, not brothers. To put two brothers who look like each other as the lead characters, I thought would be risky, but James makes you completely forget about it. He is mightily impressive.
I still can't figure out why Dave Franco is a thing, though. He is just a presence... watch his interviews and his movies and see if you can tell a difference. He just reads his lines off. He's not terrible but he can't play a character other than himself. He has no talent.