While I'd also agree that Spielberg is a greater director than Lucas, I've actually warmed up to Lucas a great deal since all my long discussions with Laz on these boards. Now, I still don't give two shits about the Star Wars films anymore, but I think you're underestimating them a bit.
So far as action set-pieces, I'd say there isn't anything in the Indy films that compares to the Trench Run, The final dual or space battle from Jedi, the Darth Maul fight in Ep. 1, the opening space battle in Ep 3, or hell, even the arena piece from Ep. 2. if I were to concede to anything from Lucas' films being superior in this debate, it would be in the action department. On top of it his films really are beautifully shot, and there isn't much in modern cinema (LOTR aside) that compares to the ambition behind the prequel trilogy. And anyone here would know how well I like to award ambition in filmmaking.
(Wow, I can't believe I'm actually defending Lucas here) But going back to the Spielberg comment, sure he's a damn talented director, but the Indy series is one of the last examples I'd point to supporting his skill. Raiders is a near-masterpiece, sure, but I still maintain that a lot of people are over-praising the rest of the series. Temple of Doom borders on "The Lost World" levels of suck, and The Last Crusade is little more than an amusing distraction in the Spielberg canon. I'd hardly call those two films showcases of Spielberg's artistic prowess, and I'd also say that Lucas' achievement even in the prequel trilogy trumps those two picture.