Both upgrade are important and the combined effect should be a faster NAS, not only for read/write speed, but also for overall responsiveness, which is not negligible when you are the administrator. Both NAS devices are marketed to home or small business users.
The LinkStation 410 ($229 2TB to $399 4TB) is a single drive NAS, while the LinkStation 420 ($289 2TB to $719 8TB) is a dual drive model. There’s also a LinkStation 421E ($149 enclosure) version which is the fastest dual-bay model at Buffalo which also features a USB 3.0 port in the front of the chassis.