Looking at the new SAM infantry and the Mobile SAM, appearantly SAM infantry is better shooting helicopters than the Mobile SAM (at least barely).
SAM infantry: 18 str + 75% vs helicopter = 31.5 (no promotion or defense bonus)
Mobile SAM: 22 str + 50% vs helicopter = 31 (lol, no promotion or defense bonus)
Also, the Mobile SAM looks like it may be a siege weapon, therefore it doesnt get def bonus, so the SAM infantry are even better at defending, and maginally better at attacking gunships. However, gunship are readily get pinch (after combat 1) then charge (after combat 2 and can gunships get charge promotion?)
The Mobile Sam is a Gunpowder Unit (see screenshot):
Mobile SAM: