YTV is a children's channel. The target age group—according to YTV—is between ages 2 and 17 (depending on the time of day and what is being shown).
Most children watching YTV are between the ages of 8 and 14. However, some of their programming is suitable for younger audiences (under 8) and some of their programming may sometimes appeal to older teens (age 15+).
- - -
Teletoon has two different target age groups.
During the day, Teletoon airs programming that is primarily targeted toward children ages 8 to 14 (so there is some competition with YTV).
However, at night time, Teletoon becomes a more mature channel that primarily targets teenagers and young adults.
The late night block features more raunchy and adult-oriented programming such as Archer, Family Guy, American Dad, Rick and Morty, etc.
Sometimes Teletoon also shows late-night movies, targeted to mature/adult audiences. For example, this month Teletoon is showing "South Park: Bigger, Longer, & Uncut" which was originally rated R in the US.
- - -
So, especially at night time, the two channels target different demographics.
YTV is a family-friendly channel (similar to Disney or Nickelodeon) that is generally considered safe for young children and teens to enjoy... whereas Teletoon is definitely not always family friendly and it's not afraid to show programming that deals with mature themes including coarse language, bloody violence, drug use, sexuality, and sometimes nudity.
So, if you ask teenagers and adults, most would probably say they prefer Teletoon.