From developer.arm.com :

Superior efficiency
Featuring an extensive microarchitecture redesign, the Cortex-A55
processor is able to achieve breakthrough performance, while maintaining
similar levels of power and 15% better power efficiency than the
Cortex-A53. The efficiency gain enables the Cortex-A55 to deliver higher
sustained performance for thermally constrained applications as well as
exceptional power savings for low-power applications.

Memory subsystem
The Cortex-A55 features an integrated, configurable L2 cache within each
CPU that achieves more than 50% reduced latency for memory accesses. It
contains an L3 cache that is shared across up to eight Cortex-A55 CPUs
within a single cluster. This new memory subsystem, which is designed
for high bandwidth, improves overall compute performance while also
reducing system power.

S905X2 vs S905X3

• The main difference is just the Core generation, Cortex-A53 vs Cortex-A55
• The X3 also adds an NPU to the SoC, which Android doesn't use.

For daily usage the average end-user will never notice the difference between DDR3 and DDR4, or the difference between X2 and X3, for a streaming device.

The most important thing is the software, is it getting firmware updates, and do those updates actually fix bugs.

I think DDR4 will help load streams faster, this is from my experience once with VLC Player with the 2 devices I have one being an 18w stb with DDR3 and a 5w dongle with DDR4. From others Netflix loads faster on Tivo4k which is DDR4 compared to GTV Chromecast presumably DDR3.