I have never owned or used a SD but own a 550. I started with it and load 9mm, .40, and .45 acp. I was brand new to reloading 2 years ago when I bought my 550. I did the same as you asked around to see what one of the 2 I should buy being on a budget wanted to same as much as possible. I chose the 550 for 2 reasons. One it does not auto index, I wanted to index manually so that I would pay more attention to the powder throw on every round and force me to pay more attention. Second was because of the dies. From what I understand the SD has certain dies you have to buy. With the 550 you have a much bigger selection of dies to choose from. Either way and dillon press is well worth the money and you will be happy. Good luck and enjoy reloading..