SSD ran out of space. Is is better to add one or replace it with a new one?

It depends on your existing system, on your budget, and your needs. Rather than typing out all the possibilities, can you just say a little more about those things to get a useful response?

Do NOT set-up a RAID 0! They are finicky, and not significantly faster on SSDs. Additionally, If one drive fails, you lose ALL of your programs and data on both drives.

Keep your Crucial M500 as a "C:" drive and buy either:
A) A bigger SSD for additional storage, or
b) An even larger, fast HDD for more storage. Get at least 7,200 RPM + 64 MB cache HDD.

Do you have a good backup system in place, like an external HDD?

Yogi

That Samsung 850 EVO 1TB is actually a better drive than the one you have. It's pretty much the best drive with a SATA connection for the money, and you'll have plenty of space.

First, backup everything you want from the existing 500 GB drive onto a third drive, which can be a USB or anything that will fit what you want to keep.

Then reinstall Windows fresh on your new SSD, which will really breath some new life into the system. I'd recommend using the new one as the primary drive because it's faster, and then use the 500 GB for storage. If it fills up, then you can put things on the new Samsung until you buy another spinning HDD for more mass storage.

To reinstall Windows, you'll need your installation key, and then to create a bootable USB for the installation process. Here are instructions on making a bootable USB in case you want them for reference: https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool

When prompted for which drive to install Windows, select every drive plugged in and press "format" and then "delete." That will make the drives totally blank and like new (remember you should have already backed up what you want). Then - and this part is important - cancel the installation and shut down your computer. Unplug the old drive, leaving only the new Samsung drive plugged in, and then go through the USB key installation process with only that drive plugged in. Doing it with both drives plugged in is bad because (for some stupid reason) Windows will not boot up unless all drives that were detected during installation are actually plugged in. That's okay until one of your storage drives dies or gets replaced, and then you'll need to need to reinstall Windows. However, installing with just the primary C: drive avoids that issue.

Once Windows is installed, shut down the computer and plug back in the old drive. It will appear in drive manager, and you can mount it as any drive letter you'd like. Boom! It will be available for storage, but Windows won't require it in order to properly boot up. You'll also be able to expand storage with additional HDDs later on if you want without risking a boot failure. Enjoy!