Which is better assassin or rouge?

Assuming you're using Colm over Rennac (the reasonable choice), a Rogue is more useful due to its innate skills. Yes, while an Assassin can pick chests or doors with an old Lockpick, he cannot steal. A Rogue can unlock things without an item and can steal items from an enemy.

Depending on your way of playing the game, either can be fine.

From a more objective point of view, though, Rogue is better because it has many pros over few cons:
Rogue gains 1 Skl over Assassin, but loses 1 HP and 1 Con
However, Rogue retains the Thief's Steal skill and also comes with a most useful Pick skill, which allows him to open doors and treasure chests without using keys or lockpicks.
Assassin, on the other hand, can still use lockpicks but loses the ability to steal, introducing a one-hit-kill move. While this OHKO looks good on paper, it actually has few to do with Thieves (which are supposed to be sneaky, weak units who avoid combat) and its high unreliability makes it far less useful than Rogue's Pick.

Colm should always be rogue, cus the other options for assassin are a million times better

Colm should definitely become a rogue. The "Pick" skill is highly valuable, whereas the "Silencer" skill I find hardly ever hits.

One thing I forgot to mention, an ocean seal can be found at Fort Rigwald (Chapter 9) on Ephraim's route.