I personally enjoyed Titan Quest more then Grim Dawn to be honest. I also have the original physical discs, but I picked up the steam versions a few years ago (so I didn't have to use the discs anymore), as well.
TQ was quite a good game for it's time, and is still enjoyable now. I personally like the flow/story etc better then Grim Dawn which feels more chaotic to me. I also felt more of a range of builds in TQ, where in Grim Dawn I felt more like I would generally only choose 1 attack type as my primary, and sometimes...sometimes an AoE or alt attak to go with it (along with many buffs etc).
Grim dawn was VERY playable with only 1 button in many cases....seemed to have less strategy needed for most fights, and especially your first time through even on elite difficulty (which there is literally no reason to play except for a bigger "challenge"....but that challenge is just giving more health to everything), it's EXTREMELY easy.
TQ on the other hand had quite a bit of challenge your first play through, had many different builds you could make, even with the same two class choices, and the class choices seemed more varied as well (to me), with more of them working well together. In Grim dawn it seems more like there are 2 or 3 possible choices for each class that really work. In TQ almost any combo could work well, at least till you got dream and just about everything seemed to work better in combo with dream.
My suggestion for new players, avoid dream. Even while playing the expansion, focus on the other combos and save dream for the end after playing other builds. It's hard to switch back to another combo once you have started using dream, or at least it was for me.