Wow, this is a big first for me - the first thread where people are discussing one of my products without having prompted the discussion myself! I feel like I've arrived

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I think 'perks' are actually advantages. They're the ones that give you free magic items, political influence, titles or a grand destiny as you develop your character. There's also a system based on them that lets you tweek the story based on the type of advantage you have, so that you can have a character turn up to give advice at a certain time or start a riot in the street.
On GT and DarkLore; Grim Tales is by its nature a much grander project than we, as humble PDF publishers, could hope to aspire to, but I think we're essentially tapping into a similar line of thinking - great minds obviously think alike

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Magic is indeed one big difference between the two, and I think GT gains an advantage based on the scale of the product. We were conscious when writing DarkLore that our magic system should be essentially the same as D&D magic, just split up into talents for the different 'equivalent' classes (scholars get Wizard magic, Devouts gain cleric/druid magic and the Destined gain sorcerer magic). The larger scale of Grim Tales means that they can step away from the restraints of D&D more than we could afford to (or at least felt we could afford to). Where DarkLore can provide a good set of rules for use in a GT game is how to adapt the spell list to make it more appropriate for dark fantasy games. There's (IMO) a good discussion on adapting spells to a darker, low magic setting. Thematically, though the difference between GT magic and DL magic is that in GT magic drains your physical well being, whereas in DL it sends you mad.
I personally think the classes are one of the strengths of DL over GT, but perhaps only a minor one. They do make a good 'bridge' between D&D and D20M/GT classes, but also I think there are some staples of fantasy character building that I don't think GT gives proper credit to, such as access to weapon and armour proficiency feats. But thats not a major thing and is fairly easily fixed.
In all GT is a good system. While I think DL has many strengths, I'm sure you can adapt them into GT with ease if you prefer to use GT rather than DL itself.
I hope Wulf won't mind me mentioning this, but we have discussed the possibility of producing a DarkLore for Grim Tales supplement, but have not had the time to work on it yet (we're in the lucky situation of having had some products commissioned so we're devoting all writing time to that at the minute). So, hoepfully in the future you may see an officially sanctioned darklore for Grim Tales which should resolve all these issues, but I have to say it might not be for a few years yet

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Cheerio,
Ben