Dell OptiPlex 5050 SFF vs. Dell OptiPlex 990 USFF
Comparison and Differences
Differences between Dell OptiPlex 5050 SFF and Dell OptiPlex 990 USFF
If you are looking for the smaller form factor the Dell OptiPlex 990 USFF is the right one for you. It is smaller than the Dell OptiPlex 5050 SFF and will fit more easily under your desk.
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 OptiPlex 5050 SFF as it is running a newer generation CPU. This is not to say that Dell OptiPlex 990 USFF is bad - it can still run some powerful CPUs, albeit being an older generation.
The Dell OptiPlex 5050 SFF has 4 RAM slots while the Dell OptiPlex 990 USFF 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 OptiPlex 5050 SFF is using the faster 2400 MT/s RAM. The Dell OptiPlex 990 USFF 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 OptiPlex 5050 SFF as it has the ability to run 64 GB of RAM. The Dell OptiPlex 990 USFF can do a bit less at max 8 GB. Most of the times, this should be sufficient for the majority of users.
In total the Dell OptiPlex 5050 SFF has 10 USB ports. The Dell OptiPlex 990 USFF is inferior in this regard and packs 7 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.
DisplayPort is crucial for any desktop machine. It is good to see that both models are featuring DisplayPorts. However, the Dell OptiPlex 5050 SFF comes with 2 ports, while the Dell OptiPlex 990 USFF has only 1. If you are planning to use multiple monitors, the Dell OptiPlex 5050 SFF is the better choice.
The Dell OptiPlex 5050 SFF comes with an HDMI port, while the Dell OptiPlex 990 USFF does not support the interface. Of course missing HDMI port is something that can be easily fixed with a good adapter (e.g. DisplayPort to HDMI).
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 OptiPlex 5050 SFF is better equipped as it comes with 3 SATA slot(s), while the Dell OptiPlex 990 USFF has 2 slot(s).
If you want the fastest transfer speeds for your SSD, an M.2 interface is crucial. Thus, if the M.2 SSD slot is a mandatory requirement for you, go with the Dell OptiPlex 5050 SFF. Sadly, the Dell OptiPlex 990 USFF has no such slot, and you will be forced to use the SATA interface instead.
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!
Having a Power Supply Unit with high power rating is important, if you are going to use components which require extra power. The Dell OptiPlex 5050 SFF comes with a decent 180 Watt PSU, but the Dell OptiPlex 990 USFF has a more powerful one rated at 200 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.