The options you are considering work completely differently.
EHLLAPI works against existing installed IBM i Access for Windows or IBM i Access Client Solutions (ACS) software. It is a very specific, solid, and well established IBM proprietary API that does not use Telnet in any way. You would need to ensure that EHLLAPI support was enabled
The UIPath internal option starts and writes to a TN5250 session, over which it sounds from the documentation as if you have little control (e.g. re keyboard mappings).
I would suggest you go with EHLLAPI if you can ( if you have a suitable IBM or third party product installed).