Well, each server has it's own "feel", I think. There are different people, and different cool shops and residences to visit, and lots of different things to explore in the wild. If you've never done any server-hopping, I would highly recommend it
Its mostly the people on it, since SMP1 was the first server I guess the oldest people are on there, and smp7 the "newest" while I play on smp7 now and I have already been playing for a while. I actually started on smp2.
Now, the question you ask has a simple answer. It is similar to the question " What is the difference between your own country and another". There is no difference in abstract ways. However from my point of view, I started EMC in smp5 server and I planned to move to smp6 because it was new. Unfortunately I just felt like I couldn't stay there. So I suggest that you go to any server smp1,2,3,4,5,6 and when time passes you will just be that part of the server. Simply the answer to your question is all are the same for a new player. As time passes you'll realize how you start becoming friends with everyone and get familiar to the environment. P.S: Welcome to EMC
Like what everyone else said, it's really the feel of the whole thing. Each server has different players, different reses, and an overall different vibe. However, some servers have special features not found on other servers. SMP1 has the graveyard, where everyone who has been banned from EMC is. SMP2 has the park, where you can take a nice walk. SMP3 has the amusement park, complete with a rollercoaster, minigolf, archery, a hedge maze, and much, much more! (note: these features were made and are owned by the admins and staff. I am not including players' residences because they are not EMC official, so to speak.)
As someone who always bounces from server to server, there are very few differences except some servers have unique places that you can visit (Ex. Amusement park on SMP3)
