I'm the MemTest86 developer / maintainer. So obviously bias.
But for what it is worth here's some comments on the differences.
When run on old 32bit systems (pre UEFI BIOS being available) there isn't much difference in them. The code base is similar, the tests are similar and they are both open source.
On newer systems (UEFI + 64bit) there are significant differences. MemTest86 was rewritten for UEFI and 64bit. The main new features in MemTest86 that aren't in MemTest86+ are,
- The code and memory addressing are now 64bit
- DDR4 is supported, including DDR4 XMP
- ECC is supported
- Secure boot is supported, MemTest86 is now code signed by Microsoft
- Multiple languages are supported. Chinese, Spanish, German, etc..
- No longer uses the Linux boot loader (but runs direct in UEFI)
- Row hammer & SIMD instruction testing was added
- Significant speed improvements
- Mouse support and GUI support
- Writing of test reports to disk (there is still no way to save results in MemTest86+)
- RAM SPD support for new CPUs & Chipsets
But as pointed out MemTest86 V6 is no longer GPL. But there is still a comprehensive free (no strings attached) version available. FOSS is nice, but having something that is current and works is important as well.