I own both (IG & ETW). Naval battles ETW are pretty much stripped from IG(including no collision).IG has board map like Shogun and MTW , and ETW dynamic like RTW $ MTW. Other differences are more developed research tree and units in ETW have morale (i am not saying that its great).
ETW feels more like improved(or redone) IG then TW game.Didn't have to much time to play , so i just tried it so far.Pathfinding problems, same old weird campaign non-AI, only 1 model for same type of infantry units.There are some good things , like slower campaign pace , etc..
One thing that really annoys me is that ,as it looks like, you going to need some sort of SDK or CA provided editor/tool in order to mod almost anything in ETW.They promised to release some "tools " down the road , but who knows if and when that is going to happen.
CA reasoning for that is "possible cheating in MP", which game is supposedly geared towards(can't say how).
So modders are pretty much at mercy of CA.
From my first impressions, i would say people are better off with EB att.Or EU3 if you are really itching for XVII century fix.
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