The PlayStation 4's is upgradeable; the Xbox One's is not. For at least the second consecutive generation (the third for the Xbox), hard drives will be offered as part of the gaming experience for two of the home video game powerhouses: Microsoft and Sony. For the Xbox line, which offered a model without a hard drive in the last generation, the inclusion of an internal HDD represents, along with its x86 processor, a return to the approach Microsoft took with the original Xbox.
Indeed, the Xbox One will load disc-based games onto the hard drive automatically. Both Sony and Microsoft will also offer access via the cloud. In fact, following up on its purchase of Gaikai, Sony plans to offer a range of gaming from the cloud to multiple platforms. This may include older titles that it cannot support on the PlayStation 4 due to a lack of native backward compatibility. If such capability is expected to work, why bother to have hard drives in these consoles at all? Indeed, hardware makers of many stripes are starting to ask that question.