Just to add a few pieces of information if it helps:
As many have said, you MUST drive both as while they are built on the same chassis, their suspensions are tuned differently with the Kia being firmer. Also, the Kia has less sound deadening materials when comparing the 2020 models (The Elantra gained more sound deadening for 2020. Something I can confirm as I've owned both a 2017 and 2020.).
Contrary to what was posted above by another member, all Elantra's purchased in North America regardless of trim are built in either Montgomery, Alabama or Ulsan, South Korea. ALL Kia Forte's sold in North America other than the GT are built in Mexico.
The 1.6T is the only GDI engine found in either vehicle. The other engine in both is a 2.0L MPI only engine connected to a CVT (2020 only on the Elantra) developed by Hyundai/Kia that uses a chain instead of a belt like a traditional CVT.
Last, the Elantra comes with more safety features standard in the lower trims than the Forte. Specifically, Blind-Spot Monitoring and Rear Cross-Traffic Alert are standard from the SEL trim and above. Those don't come on the Forte until the GT-Line trim.