New Computer: Ram Questions

I have decided it is time to upgrade my 8 or so year old Dell Dimension 8400 that's running Xp Pro. I'm looking at referb desktops on Dell's Outlet site. I'm looking at two Dell XPS's, both the same, except for ram. Both run Windows 7 professional (I don't want Windows 8), 2TB hard drive, Nvidia Gforce GT 640 video card, and a DVD +/- RW drive (I don't have any blue rays to need a blue ray drive) The first one has 12GB Dual Channel DDR3 SDRAM at 1600MHZ. The other one has 16GB DDR3 UDIMM Memory, 1600MHz, non ECC (4 x 4GB DIMMS). For ease of discussion, we can call them 12GB and 16GB. Other than generally more is better, I don't know a whole lot about RAM. I know DDR3 is double data rate type 3. And non ECC is non error correction code. Other than that, Im kinda lost. What is the difference between these two? Is one "better" than the other? The only difference to me right now is that the 16GB is 4GB more than the 12GB and $100 more than the 12GB. My Dimension has 4 slots for memory. I believe it originally had 1GB, with two 512KB sticks. It now has 4 1GB sticks, even though it can only read 3.25GB available. Would I be able to upgrade either of these the same way? Any kind of info or help with this is greatly appreciated.

yes they are both more or less exactly the same, except 1 has an extra 4GB stick installed, probably just a cost saving feature of the pc maker, i can't think of any other reason why they wouldn't install the usual 16GB if they installed more than 8GB of memory in a pc unless there was a hardware fault. today most motherboards are capable of using a max of 32GB, that's 8Gb sticks in each of the four slots on the motherboard, so yes they could be upgraded with more memory if you chose but we'd need to see the full specs of both machines to confirm.