To make a game have more feeling like the old planescape, I decided to give a free feat to everyone. They,re supposed to work like this:
Primer
+4 vs. banishing (if a spell also does something else, banishing possibility happens only if the victim fails it's save by 4)
Planar
Portal spotting (find portals like rogue finds traps or elf finds secret passages)
Knowledge[planes] as a class skill
This makes a few problems:
1) On higher levels, portal spotting loses it's use (anyone can just teleport around) and protection from banishing helps a lot
-I don't know anything about this because I never played on such high levels, this is only the advice I've been given
2) What about spellcasters? I can give any free knowledge to a cleric but wizard has already got all of them. What can I give to a wizard?
Currently, everyone likes the "planar" feat better because there's no true wizards and they could all use knowledge[planes] to know things about outsiders and enviroment, but I'm still scared of disbalancing the game in the future
Should I add something weak but enjoyable to the planars (altough, "not a primer" is almost like a courtesy rank in the planes 
) just to make things even?
, I'm definitely using this
Primer
+4 vs. banishing (if a spell also does something else, banishing possibility happens only if the victim fails it's save by 4)
Planar
Portal spotting (find portals like rogue finds traps or elf finds secret passages)
Knowledge[planes] as a class skill
This makes a few problems:
1) On higher levels, portal spotting loses it's use (anyone can just teleport around) and protection from banishing helps a lot
-I don't know anything about this because I never played on such high levels, this is only the advice I've been given
2) What about spellcasters? I can give any free knowledge to a cleric but wizard has already got all of them. What can I give to a wizard?
Currently, everyone likes the "planar" feat better because there's no true wizards and they could all use knowledge[planes] to know things about outsiders and enviroment, but I'm still scared of disbalancing the game in the future
Should I add something weak but enjoyable to the planars (altough, "not a primer" is almost like a courtesy rank in the planes
) just to make things even?
I'd say overall power balance between the two feats is fine. If a prime survives long enough for a +4 versus banishing to be useful, more power to them - and players who insist that 'portal spotting is useless because everyone can teleport anywhere anyway' need a quick trip to a demiplane or Abyssal layer warded against teleportation to snap them out of their complacency.
As for the 'what to replace Knowledge (the Planes) with' issue, I'd actually recommend the Search skill, across the board. Neither bards, nor clerics, nor wizards (the three base classes that get Knowledge (the Planes) as a class skill) receive this skill, and granting it to them as a class skill makes portal searching that much more useful. Something like Gather Information or Sense Motive could also work for wizards (though a bard already receives both of these, as well as all Knowledge skills - s'why I thought of Search. Oddly enough, the SRD I'm referencing doesn't list it as a class skill.)
Hope this helps!