Dell OptiPlex 390 DT vs. Dell OptiPlex 7020 MT
Comparison and Differences
Differences between Dell OptiPlex 390 DT and Dell OptiPlex 7020 MT
If you are looking for the smaller form factor the Dell OptiPlex 390 DT is the right one for you. It is smaller than the Dell OptiPlex 7020 MT 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 7020 MT as it is running a newer generation CPU. This is not to say that Dell OptiPlex 390 DT is bad - it can still run some powerful CPUs, albeit being an older generation.
The Dell OptiPlex 7020 MT has 4 RAM slots while the Dell OptiPlex 390 DT 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 7020 MT is using the faster 1600 MT/s RAM. The Dell OptiPlex 390 DT 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 7020 MT as it has the ability to run 16 GB of RAM. The Dell OptiPlex 390 DT 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 7020 MT has 10 USB ports. The Dell OptiPlex 390 DT 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 OptiPlex 7020 MT has a DisplayPort. The Dell OptiPlex 390 DT does not have one? Don't worry though, nowadays video adapters are cheap and you can get almost any combination of conversion types.
The Dell OptiPlex 390 DT comes with an HDMI port, while the Dell OptiPlex 7020 MT 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 slots are located on the motherboard and it is where SSD, HDD and Optical Disk Drives are connected. Each of the two models being compared have 4 SATA slot(s). Keep in mind that having too few SATA slots might limit your options for connecting additional memory drives.
M.2 SSD interface allows you to transfer data with higher speeds compared to the old SATA interface. Unfortunately, neither the Dell OptiPlex 390 DT, nor the Dell OptiPlex 7020 MT 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!
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 390 DT comes with a decent 250 Watt PSU, but the Dell OptiPlex 7020 MT has a more powerful one rated at 290 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.