I know what you mean Gerry, I'm building an O10.5 atm. I have absoluteley no need to - just enjoying doing it. (My excuse was that I had lots of bits left over)
THe fane unit does look good, it kinda matches the Mid range horn and give a nice symmetry, although if you decide to put on a grill it doesn;t matter what you use visually. I opted not to grill mine in the end so I could bore people with the details of my hardwork when they say "that's an unusual looking cab..."
As you can see the OT15 fits perfectly into the boot (trunk) of a PT Cruiser, although TBH most of the time I have the seats folded down for other people's kit and speed, but it's nice to have the option of being able to take my own rig and 4 other people to a gig.
I've said it before but I still don't know how I ever managed without this cab, and dread doing gigs without it.
As for what to build, well I've not heard an O12, but I'm sure most peoples recomendation will be too build both and see which you prefer. And I can't argue with that
R