Dell OptiPlex 790 USFF vs. HP ProDesk 490 G2 Microtower
Comparison and Differences
Differences between Dell OptiPlex 790 USFF and HP ProDesk 490 G2 Microtower
If you are looking for the smaller form factor the Dell OptiPlex 790 USFF is the right one for you. It is smaller than the HP ProDesk 490 G2 Microtower 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 HP ProDesk 490 G2 Microtower as it is running a newer generation CPU. This is not to say that Dell OptiPlex 790 USFF is bad - it can still run some powerful CPUs, albeit being an older generation.
The HP ProDesk 490 G2 Microtower has 4 RAM slots while the Dell OptiPlex 790 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 HP ProDesk 490 G2 Microtower is using the faster 1600 MT/s RAM. The Dell OptiPlex 790 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 HP ProDesk 490 G2 Microtower as it has the ability to run 32 GB of RAM. The Dell OptiPlex 790 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 HP ProDesk 490 G2 Microtower has 8 USB ports. The Dell OptiPlex 790 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.
Unfortunately, only the Dell OptiPlex 790 USFF has a DisplayPort. The HP ProDesk 490 G2 Microtower does not have one? Don't worry though, nowadays video adapters are cheap and you can get almost any combination of conversion types.
None of the models has an HDMI port. If your monitor supports only the HDMI interface, you will need to buy an adapter. Such adapters are cheap so nothing to worry about.
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 HP ProDesk 490 G2 Microtower is better equipped as it comes with 3 SATA slot(s), while the Dell OptiPlex 790 USFF has 2 slot(s).
M.2 SSD interface allows you to transfer data with higher speeds compared to the old SATA interface. Unfortunately, neither the Dell OptiPlex 790 USFF, nor the HP ProDesk 490 G2 Microtower 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 790 USFF comes with a decent 200 Watt PSU, but the HP ProDesk 490 G2 Microtower has a more powerful one rated at 300 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.