[Warning: VERY long and overly psychologic comment. ]

I voted for similarities, but probably the differences are more evident... I will explain better.
You should know by now that I'm totally mad about Orochimaru and Kabuto , since the very first time I saw them... I first stumbled upon some episodes when they started airing the show on Italian TV, and I still giggle remembering those times when I used to think to myself things like: "oh my, another shonen anime? It looks just like all the others! We definitely don't need another one..." and "Luckily I'll never start to really watch it, these guys have names I could NEVER remember..." THEN that episode popped out, the one before the third test of the chuunin exam, when Oro and Kabu are talking about Sasuke and their evil plans, and... I heard bells ringing. At first, I was probably fascinated by Kabuto's smirk and reflective glasses, that reminded me so much of Kevin from the Sin City movie (yes, looks like I have a weak spot for ninja-skilled crazy boys wearing glasses and whose name starts with "K"), and about Orochimaru... oh, his bossy attitude and wonderful EYES (plus, I've always loved reptiles and snakes in particular... ^^).
I was lost forever. XD

Back to the main topic: they were the main reason why I began to love Naruto and to read the manga. Their mysterious characters had me more and more concerned, to the point of feeling a great empathy: I mean, they've always been represented as pure villains, doing harm just for the fun of it, but I'm sure both of them must have suffered greatly in their lives, they've been alone, and that's what led them to what they have become . That's the main reason why I chose the "similarities" option: I've been through bad times in my life, when I felt sad, alone, betrayed, and nonetheless I felt like I was the "bad guy" in the eyes of some people. Now I see I was deaf, I couldn't see many things and made many mistakes, things I'm not proud of, but that was because back then I felt like suddenly I had no more friends beside me, no one to talk to, no one who could understand. That's something that can make you go crazy. And this is when I felt I had to bear a secret suffering that I'm sure my favourite villains have to deal with, too...
About differences, sometimes I even envied Kabuto's coolness and his ability to hide his feelings, something I've never been able to do. And sure he's never been all that happy inside, as well...

That's why I love so much those mistreated characters, and I can't stand the way Kishimoto makes them look only like two crazy weirdos with no soul inside while other characters get all the explanations and sympathy, especially when I think that sympathy is not all that deserved. But that's just my opinion, of course.
(And yeah, of course villains are supposed to be villains, and for example Naruto was terribly alone as well, but he's the hero because he could find his strength and will: it makes sense. But please, Kishi, PLEASE, give a bit of humanity to those poor fallen ones too... )