Spielberg is probably my all time favorite director,but with that said,I prefer The Thin Red Line.
Private Ryan is a really good movie,but after the opening,which is extremely visceral,there are big dull spots that have you waiting for another battle.
The Thin Red Line is a fucking masterpiece of a war film that asks questions that I haven't seen in any other war movie.While Private Ryan shows you the horror or war,in probably THE most realistic way ever captured on film,that's all it does.
The Thin Red Line shows you the inner struggles and focuses on the psyche of the soldiers,which I found to be much more interesting.
I think Spielberg is probably the greatest director of all time with hands down the most impressive resume of any other director ever,but between these two movies,Terrence Mallick has crafted a much more thought provoking film that imo has been severely overlooked.