You'll want to prioritize the trophy skill, at least until you get it to 2/3 so you dont get boned by the absolutely tight economy. You'll want to get 3/3 eventually, sooner is better, but it can wait a bit. Otherwise you're free to go with what you want.
Yeah, I already reached 1/3, I knew it was important from playing Gothic 1-3 [
finishing 2 of them] and Risen 1-3.
Wondering if the "extra XP" skills are worth it, too.
Late as hell to this, and probably no longer relevant, but the answer for the extra XP skills is mostly "no". You can get the book reading one from an amulet, which you can just equip when going through them. You can get the monster killing one as well, but truth is that they give little extra XP to really "make up for it". If you're going for a cold playthrough and don't mind chugging elex potions, then yeah sure, since it also raises your 'charisma' stat, but in terms of getting more XP to be "worth it", no. The extra attribute at level up however is worth it if you get it early enough, unless you specifically plan to be a low INT build.