Alienware X51 vs. Dell XPS 8900
Comparison and Differences
Differences between Alienware X51 and Dell XPS 8900
If you are looking for a more modern desktop, then you should go with the Dell XPS 8900 which was released in 2015. The Alienware X51 is older and entered production in 2011.
If you are looking for the smaller form factor the Alienware X51 is the right one for you. It is smaller than the Dell XPS 8900 and will fit more easily under your desk.
Processor
Usually the newer the generation of the CPU, the better the performance and efficiency is. If your budget allows it, you should go with the Dell XPS 8900 as it is running a newer generation CPU. This is not to say that Alienware X51 is bad - it can still run some powerful CPUs, albeit being an older generation.
Memory
The Dell XPS 8900 has 4 RAM slots while the Alienware X51 has only 2 slot(s). This is not something to worry about. Just make sure you take it into consideration when planning how much RAM you are going to use.
The Dell XPS 8900 is using the faster 2133 MT/s RAM. The Alienware X51 is slower by supporting RAM speeds of up to 1333 MT/s. Additionally, if you will be needing a lot of RAM, better go for the Dell XPS 8900 as it has the ability to run 64 GB of RAM. The Alienware X51 can do a bit less at max 8 GB. Most of the times, this should be sufficient for the majority of users.
Ports
In total the Dell XPS 8900 has 10 USB ports. The Alienware X51 is inferior in this regard and packs 8 USB ports. In any case, if you miss USB ports, you can always purchase a USB hub. None of the desktop models has a USB 3.1 port (10Gb/s), so if you need a fast connection to your peripherals, like external Solid State Drive, you will need to look elsewhere.
Unfortunately, only the Dell XPS 8900 has a DisplayPort. The Alienware X51 does not have one? Don't worry though, nowadays video adapters are cheap and you can get almost any combination of conversion types.
In case your monitor supports HDMI only, you will be happy to hear that both models come with an HDMI port.
SATA ports are placed on the system board and it is where you connect your HDD, SSD and Optical Disk Drives. More SATA slots will allow you to have more drives running at the same time. In this regard the Dell XPS 8900 is better equipped as it comes with 5 SATA slot(s), while the Alienware X51 has 4 slot(s).
M.2 SSD interface allows you to transfer data with higher speeds compared to the old SATA interface. Unfortunately, neither the Dell XPS 8900, nor the Alienware X51 has one. However, you can still use a SATA SSD drive which should give you satisfactory performance.
Make sure that the available M.2 slot supports the PCIe (also called NVMe) interface. This is important as there are M.2 slots which support only the inferior SATA III interface. For comparison, the SATA III interface has max speeds of 6Gb/s, while the PCIe 3.0 x4 will support speeds up to 32Gb/s!
Power Supply
Having a Power Supply Unit with high power rating is important, if you are going to use components which require extra power. The Alienware X51 comes with a decent 330 Watt PSU, but the Dell XPS 8900 has a more powerful one rated at 460 Watts. Choosing either one depends on your specific requirements and use case. Also don't forget that some models might have more than one PSU option - try to always get the most powerful one, especially if you plan to upgrade to a powerful GPU.
Motherboard
