The Arp9, for the money, is quite spectacular.
And as @Adolf Hamster says, with the drum mag is epic.
With the beastly noise it makes, high RoF and 1500 rounds, it can suppress really effectively.
Then you can swap to mid caps and stick a small tracer on and go for CQB fun.
The X9 mags will fit the arp9 and are higher capacity.
That being said, I'd like to get hands on with the x9 to see what the difference is.
From playing quite a few games with the ARP, I would have to say the X9 would have to be very special to make me want to swap.
