Main differences between 3520 and 7520?

Google is failing me as far as directly comparing these 2 models, however from what I gather... the 7520 has more ports (including one for a docking station) and has more RAM / HDD slots that can be used at the same time.

I think my main question is... are the bodies the same? They look very similar outside of some visual cues. I have a 7720 I use for work and I really like it, but I could live without the expand-ability if I get the same build quality from the 3520.

The other core concern is the TB3 port and if there is any limitation that exists on the 3520 that doesn't on the 7520? Or any other surprises that would make the 7520 the obvious choice.

Thanks for any input!

 

7520 is a smaller version of the 7720 so you can imagine what that body is like. 3520 is more like a Latitude body, mostly plastic.

 

Ahh ok, it's hard to tell that just from pictures. Although I think the 7720 I have is also mostly plastic, it feels really high quality. Also one other thing I just noticed is you get 3yr warranty with the 7 series vs 1yr warranty with the 3 series.