They are pretty clearly the same size from the pics IMO. The PCI-E slot is a certain length. You should be able to get a very good idea of how large these cards are from how much it extends past the slot.
Why do you need such a short card? The pics I'm seeing of the Dell Inspirion 531 show no need for cards this short.
Scaling it was how i came down to these 2 cards but images arent always correct from what i've found . It is very close to the bank of sata ports and can only be 190mm max and not too big a fan either.
How do you attach a photo to this forum as i have a pic of it ??
All of the information I am seeing about the motherboard in your computer does not agree with the numbers you are giving. The bank of sata ports is several inches beyond the PCIe x16 slot while you seem to be fretting about the exact measurements of cards that obviously won't even come close to being near them from the pictures.
This is supposedly your motherboard
I doubt the cards you are looking at would even make it to underneath the RAM slots, much less be an issue for the SATA ports on the other side of them.
There are sites to upload images like tinypic or imgur but it doesn't matter if that is your motherboard. As I said the cards you are looking at will not even come close to the SATA ports. You can in fact use even longer cards.
I do believe the cards you are looking at are actually shorter than the one you already have installed. All of this does not matter however because as rolli points out there are SATA cables with right angle connectors that should make this a non-issue. This one is half the price however; http://www.amazon.com/StarTech-18-Inch-Right-Angle-SATA...
I think you should only need one.