G&G combat machine or DBoys full metal m4 s-system?

i have a dboys ak-74m but i am now looking for an m4 style gun. i have narrowed it down to the two in the title. i was wondering if anyone had any problems with either of them. the G&G is $140 and is nylon reinforced polymer and doesn't have an RIS while the dboys is $160, full metal and has an RIS. i was mostly wondering how sturdy the G&G is and if it can withstand being used on a field for 7 hours at a time every other week and how long is both of the guns' avg. lifespan is before i need to replace something in the gearbox. thanks, in advance.