I have both, Defender was the 1st game i ever collected. I have had it since 1993 and i'll never sell it. I got a Stargate about 2 years ago, i always liked the cabinet better the art work is awesome! BITD when i saw a Stargate i got pretty excited because i didn't get a chance to play them much. There always seemed to be a Defender in the Arcades i went to but not very often a Stargate.
Personally I have always played my Defender more, a lot more. I like Stargate and i agree some of the graphics are smoother, but Defender is just cool'r it was the 1st. I find Stargate a little easier to play as well at least on the lower leves.
If i had to choose between the two i would go for Defender it's more iconic. It blows me away that Defender doesn't get more love on KLOV.
Since its release, it has become one of the highest grossing arcade games ever, earning over a billion dollars. (eough said)
Honestly you can't go wrong with either, they are both great. Next you will need a Vector, go for Star Castle!
One last thing like mentioned about the game will "bomb" at your Arcade parties. The last few i have had I always make an annoucement, being kind of goofy i always ask "why isn't anymore playing my fav game"??? they always answer the same, too many buttons, don't know how to play it, blah blah, i in return say who wants to learn? There is always one guy that is a gamer and has probably heard of Defender or if my age played it 30 years ago so says yeah teach me. With that a crowd will gather. I give them some quick tips dying off pretty quick as i go then ask do you want to watch me play a game? The crowd will watch and laugh the game is quick and gets crazy. But don't expect Defender to be played a lot, you have to show people and do all the "work" in the relationship when it comes to the might Defender! LOL
But at least you will leave that one "gamers" standing at that 30 year old machine trying to play muttering "so many buttons, so many buttons"