QUOTE(churl @ Feb 9 2008, 11:53 PM) [snapback]509461[/snapback]
I own both, and I've included comparison photos. I think both are great pens. The Prera is lighter, and just a bit shorter when cap is posted. The nibs are substantially different. The Prera's F is more like XXF, while the Sapporo's F is similar to other Japanese F's. The Sapporo is also wetter. If it makes a difference, the writing on the card was with my Sapporo (Mont Blanc brown ink).
I don't use my Prera much because it's too fine for me, but I didn't have trouble with the length. The Sapporo is an everyday workhorse pen for me, and I love it. But if you're really concerned about length, there's the Sailor Professional Gear. Hope this helps.
I forgot to mention that - the Prera F is quite a bit finer than the Sapporo F, but I don't know if it's because it's actually physically smaller, or because the Sapporo is a wetter writer. But my experience is the same as Churl's. I use the Prera for margin notes, and while reading papers, but I use the Sapporo for actually writing.