Where do I have a better chance of getting in? Early action to Stanford where I am a legacy, or early decision to Northwestern where I have no connections. I have an unweighted 3.8 GPA (weighted 4.3) at a challenging private school, 34 ACT, 800 SAT Math 2.
To chance you more accurately, we’ll probably need to get a look at your ECs and other outside of school activities. However, just statistics-wise, Northwestern ED will probably give you a slightly better shot, just because Stanford is so insane and even having legacy may not really boost your application a lot. That being said, I think you should apply to the school you really love more, legacy or ED aside. Your passion for your top choice will shine through more genuinely and hopefully admissions officers will see that.
Legacy at Stanford gets you a second reader to see if they concur. How much does that help your application, considering 3/4 of the admits have a 4.0?