i believe the short answer is the 2.5 is the base model. (upgrade if you want more). But I would imagine, like me, you might be looking into your options...
The 2.7 model has the tray/cabling /drive. Searching the internet, I believe the 2.5 model can be updated to add a SSD or even dual 2.5" HDD in the 3.5" bay with the proper parts.
1) The easiest way to go is get the iMac with the SSD to begin with. (too late for me)
2) add a SSD in the same 2.7 model's "location" after securing the proper third-party parts.
3) another option is to swap the 3.5" HDD with a dual 2.5" drive tray to add a 2.5"HDD+SSD or (2) 2.5" HDDs. there may be temp sensor issues.
I'd love to have a second internal drive. I'm going to wait to see how the option pan out.