Grim Dawn has an atmosphere that is a lot more like Diablo 1 (and also close to D2), it has closer spawns, it has factions. TQ series is a little brighter and more colorful, it's a little more spread out for spawns, but the skills system, the interface which provides you with ALL THE INFO you could ever want, and the combat are the same process.
If you like it, well you know 92% of the game mechanics of Grim Dawn. The simplest way to go about it, I suppose, would be to get Titan's Quest and Immortal Throne first (or this Anniversary Edition if you have not already got TQ/IT with the 1.17 sd community patch).
Yeah, I think you'd have to say both really.
