The question comes down to the operating system you plan to run.  If it it's Linux, either will work but the newer 7920 is a better idea.

If it's Windows, note that only the 7920 has CPU support for Windows 11, so scratch the 7910 off the list for that.

The major difference between 7910 and 7920 is the flexbay.  It's a modular bay designed for flexibility, allowing users to easily add or replace storage HDD/SSD with support of the high performance NVMe and U.2

Although some earlier 7920 with Skylake may not officially support Windows 11, it certainly is a better choice over the 7910.

@ejn63​ hi I will be only running windows 10 or 11 am I correct that 7910 can only run windows 10 and not 11 ? If this true why

@Chino de Oro​ just to get this straight windows 11 can be installed in 7910 only 1920 without skylake and what is skylake how do I know 7920 I am buying has skylake thanks 

Please note the word officially support.  You can install Windows 11 on either systems, but the 7910 does not have Windows 11 drivers from Dell.  Some bypass may require for the latest Win 11 installation.

Skylake is code name for 6th gen processors, Kaby Lake is 7th gen processors and so on.  Windows 11 requires at least 8th gen processors as official support.

Hardware or software support, the 7920 is still a better choice over the 7910.

https://www.intel.com/content/www/us/en/products/sku/123541/intel-xeon-gold-6132-processor-19-25m-cache-2-60-ghz/specifications.html

The 7910 supports Haswell and Broadwell processors, also known as 4th gen and 5th gen processors.  That was why it is not officially support Windows 11.

@Chino de Oro​  so I am looking at this Dell to buy it’s 7920 model but it has Dual Xeon Silver 4110  this processor when I google it it says product formerly sky lake but seller says it has windows 11 installed I thought you can’t install skylake without modification or am I missing something here thanks 

Your main query:  which is the better one for your usage, Precision 7910 or 7920?  The answer is certainly get the 7920.

Any other concerns can be resolved with better clarification, including hardware upgrade.  Dell releases Precision 7920 sometimes back in 2017, when Windows 11 didn't come out until 2021. 

Example of issue, some users with the earlier formerly Skylake processors have been waiting for Win 11 version update but Windows 11 24H2 never came.  The assumption was unsupported hardware.  Unlike the 7910 needing bypass and lacking of drivers, Dell support site does have Win 11 drivers for 7920.  Users can perform Win 11 installation on 7920 without any bypass or workaround.

Since Dell stopped selling the 7920, many processors value dropped to nothing.  For example, Xeon Gold 6138 was retailed for over $2,600 USD and now can be had for just $15 USD.  Therefore, your plan of building from barebone can be realized.

If you have access to US market, widen your choices and look for 5820, 7820, and 7920 as they are within the cycle of upgrade transitions and off leases from businesses.

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