The PSA JAKL VS MCX. They have similar AR15 alike features, but they are very different in term of designs. While some advocate for the JAKL due to its suppression capabilities and value, others lean towards the Spear LT for its perceived robustness and reliability.
Right off the bat, Sig MCX is an expensive firearm compared to the PSA JAKL. The Sig MCX has been on the market for some quite time now, and the latest iteration of the MCX Spear LT has improved many things from its original release. Some users argue for the Spear LT's superior robustness and reliability over the PSA JAKL, considering it a better option if budget allows.
PSA JAKL VS MCX Pricing

As far as dollar value considerations. The PSA JAKL is much more affordable and the complete upper receiver can fit most standard AR15 lower receivers with some minor changes.
The MCX upper can also work with a standard AR15 lower receiver with an adapter, but you have to pay a premium for a Sig MCX.
Functionality
The Sig MCX Spear LT still uses the traditional AR15 style charging handle, and the PSA JAKL uses a side charging handle. The best part about the JAKL's charging handle is that its none reciprocating unlike a FN SCAR 16.

This means for tactical training, the shooter can bring the gun closer to the ground without scraping the charging handle.
As far as rail system and customization goes, both MCX and JAKL have plenty of mounting spaces for lights, lasers, grips and other attachments.
Calibers
The Sig MCX and PSA JAKL both offer 5.56 NATO and 300 blackout. The MCX Spear LT also has the 7.62 x 39 version available.
PSA JAKL Problems
The PSA JAKL is still a fairly brand new upper receiver that features a long stroke piston rod, which functions like an AK rifle. The reliability of the JAKL just off initial 1000 rounds shooting shows its potential to last for a long time if it's maintained correctly.
The only noticeable problem with the JAKL is not dialing the gas block which can cause cycling issues if there isn't enough gas. So if you run into this issue be sure to readjust the gas block.

Some users have reported fitting issues with the JAKL on some AR15 lower receivers, and it was discovered that PSA did in fact have some issues with a particular batch of JAKL, and newer models have resolved these issues.
Some users have reported shell ejection problems with the PSA JAKL where the bolt fails to load the next round even after changing magazines. When tweaking the gas setting can fix the issue.
Sig MCX Spear LT Problems
The Sig MCX Spear LT is a premium product coming from Sig Sauer, but there has been many concerns of Sig's quality control and customer service over the years.

Given the Sig MCX Spear LT's high price point, many users expressed their disappointment with the quality issues encountered. There was an expectation that a rifle in this price range should have impeccable quality and no such issues.

Referenced TheFirearmBlog
As far as addressing some of the physical problems with the Sig MCX Spear LT, there are some common problems:
- Wobble barrel / flexing too much
- Barrel bolts weren't torqued to spec
- Laser device may not hold zero if the handguard isn't installed properly