DS218+ is a cheaper dual-bay NAS with an Intel Celeron J3355 CPU, while DS418 is a slightly more expensive quad-bay NAS with a Realtek ARM based RTD1296 CPU.
A quad-bay NAS offers more flexibility for hard drive selection, however its ARM based processor and soldered onboard RAM means there's no hardware upgrade, virtualization support and may become a bottleneck when running multiple add-on packages.
The dual-bay NAS has more functionality thanks to the (relatively) power X86 CPU, but its limited by its only two bays, meaning you have to run it with a very basic dual-drive RAID setup, thus nullifying its performance advantage for VM and add-on.
Ideally (if budget allows), you should go for things like a DS418play. But if you have to choose between the two you've mentioned, I would go for the DS418 since it will better function as a storage device with few add-on installed.
25 minutes ago, TicTacFloof said:
Also, is 2GB of RAM enough for a 10TB RAID array ?
Yes if you only use it as a storage device. And if you do need to add more, DDR3 price has dropped dramatically. Not all Synology NAS are upgradable anyway.