256G Flash SSD vs 2TB Fusion - internals

Sorry if this has been covered, I've searched quite a bit and found lots of recommendations, but my question is specially regarding how the computer is built by apple in these two options and I didn't see that...

I understand that if I order the 2TB Fusion, it comes with a 128GB SSD Stick on the logic board and a 2GB HDD in the drive slot (1TB Fusion comes with a 64GB Stick and 1GB HDD.) I can keep it fusion or "defuse" it. For the same price (setting aside, memory, processor, graphic card) I can order the 256Gb Flash version.

It this case is that all on a SSD stick on the logic board with an empty hard drive slot or some combination of 2 SSDs that use both spaces? Also would the answer to that question be the same if I went with the 512GB Flash SSD BTO option.

Just wondering about how it is built when ordered and how that impacts my options for upgrading later...

 

Thanks much for the info! So, when ordering, really a question of the trade off between a bigger PCIe SSD with no HDD or a smaller SSD with a HDD and then leave fusion or defuse...

So while it's possible, getting apple 256 or 512 SSD and then adding internal storage HDD myself or even creating my own fusion with the bigger apple SSD and adding a HDD are probably steps too far, needing cage and cables and such. I can always add fast external without cracking open the iMac and these other work arounds....

As always, help here is always useful and much appreciated!

 

You should add a second hard drive instead of a hard drive.
Not really sure what you mean by this, only room for one hard drive in an iMac.

Follow my guide here: http://forums.macrumors.com/threads/guide-replace-the-hard-drive-in-your-27-imac-slim.1979747/
Thanks, actually, I did read that prior to posting, it was very helpful, as were the ifixit instruction sheets and videos, OWC website and some other youtube videos.

My question is really more about which build to order, and how they are constructed...I get that I can replace the HDD with a SSD if I order one that has one in it. But if I want more than the 120GB SSD stick that comes with a 2TB Fusion drive model then I have to BTO the 256GB SSD or larger version. Those have an empty Drive slot, if I'm understanding the answers I'm getting from you all. Sure, if I was to bother cracking it open I would add a SSD not spinner HD.

For roughly the same amount of $$ (depending on which model is ordered, maybe +/- $100) I can get a 2TB fusion, (120GB SSD stick + 2TB HDD) or a 256GB SSD Stick (empty drive) that's the trade off I think.

Then if I go with one with an empty drive, it becomes a question of value, time and money to crack it open, buy the cage (maybe), cable and other kits and parts, + whatever drive I want to put in...vs just putting something external together.

sorry didn't do the quoting right...thanks agin for the thoughtful responses!