ICS and G&G are both great companies, both Taiwanese and both somewhat TM compatible, what i'm trying to say is, if you go for the ICS you'll be amazed by the split gearbox and the cool forward assist used as a spring decompression but one downside is the only thing that will be TM compatible would be the mexhbox INTERNAL, no hopup the externals may be compatible but may require some minor fixations
G&G (if your buying a classified one) to my knowledge doesn't have the great metal body compared to a ICS but is 100% tokyo marui compatible (I think). leaving you room for tons of external and internal upgrades.
So basically, up to you personally. Watch some youtube videos.