Just keep in mind apart from what @F737NG already said, the 600 and 900 version doesn’t have any cargo airframes as well if that’s something you are interested in.
600 has a single type of airframe
700 has 3 main types of airframes, pax, cargo (bdsf) and bbj
800 has 4 main types of airframes, pax, cargo (bcf), cargo (bdsf) and bbj
900 has 2 main types of airframes, pax and pax er.
700 pax, bbj, 800 pax , bbj and 900 er has two types of wingtip options - blended winglet and split scimitar winglets.
Now it’s all laid out in front of you. I would pick the 600 if I am not too serious about real world ops or the 800 if I want different types of ops including cargo. But keep in mind their flight behavior and feel is probably quite different. Since I am not a real world pilot, I cannot tell you how realistic those differences are here in sim. I enjoy the 800 quite a bit, it being quite long, you have to keep tail clearance in mind during rotation and flare. Same goes for 900 but i don’t own it. I enjoy the 600 as well, it a fun little plane.