OK guys here's the skinny, I installed the larger one for a friend and ran it through it's paces with an antenna analyzer. I found that the long adjustable radials when tuned (using an SWR meter for you CBers out there) near their full length resonated fairly well for using it on CB but it was a POS on VHF low band which was it's intended purpose. When those radials were fully retracted it resonated there perfectly. That adjustment BTW had no effect on other resonances for the higher frequency ham bands and no effect on scanner receive except for VHF low.
The smaller one is a pretty much run of the mill discone which is a POS on VHF low but resonates fairly well on the ham bands above 144MHz. Other than low band it makes a pretty good scanner antenna but like them all stinks on 800Mhz. The choice is clear, the big one for CB or low band and general scanning or the small one if you don't listen to CB or low band.
Final note and I can't stress this enough, for 800MHz nothing beats a dedicated 800MHz antenna, accept no substitutes. Do we have to go through the coax issue AGAIN? (;->)
"Also, if there is anybody out there who's tried both inside a roof space and on top of the actual roof, has it made a noticeable difference?"
Yes; noticeable but not critical, you could do a lot worse.