Which is better, to access the router using telent or SSH and why ?

SSH is better of accessing network devices because it encrypt the traffic between the admin PC and the target device while telnet send the traffic including the passwords in clear text , so when you can use both give the preference to SSH

SSH is beter than Telnet because it is a secure way , it transfer an encrypted data ..

Its a trade off between fast connection and better security. In terms of security, SSH is better whereas for a fast access telnet is used.

Ssh Is secure shel which data is send through encrypted format..

ssh is more secure than telnet 

use Telnet only if SSH is not available, never compromise security.

If your access the device on premises then prefer telnet 

off premises ssh

SSH  more secure the telnet , and your data encrypting in SSH method ,   you can use wireshark to see the deference 

Telnet is not secure Protocol. Telnet is blocked in Most Secure Network. SSH is  better Option!

In telnet, the password is in clear text but in ssh it is encrypted.

SSH being secure is best although it is slow, but most of the times company needs security rather than fast connection.