The Rollei 35 RF is a rebadged Cosina Voigtlander Bessa-R2. It's an all-manual 35mm rangefinder. The batteries power only the meter, which is the same as that in the R and the R2. The only modifications that differentiate the Rollei from its Cosina cousin are the frame lines and a different paint scheme, body covering and display box.
The Rollei uses a non-electronic shutter, while the Zeiss Ikon uses an electronic shutter.
Because it's an all-manual camera, there is less to go wrong.
However, because it's a Cosina product, there is the possibility of rangefinder misalignment -- an issue that seems to bedevil too many of its rangefinders (if complaints on Internet forums are valid). Indeed, my own Rollei 35 RF has a slight rangefinder misalignment. But it's not enough for me to send it in and have it fixed.
The camera is reliable, although it hasn't seen heavy use.
Regarding the Sonnar, it's a very sturdy lens of brass and steel construction. Focusing is via a tab. This lens works much better on the Zeiss Ikon than on the Rollei, because the rangefinder arm is too heavily sprung on the Rollei (and the other Bessas, as well). That means focusing takes more effort. It's smooth from infinity to 0.7 meters but stiff when focusing in the other direction.
On the Zeiss Ikon, focusing this lens is very smooth and requires only a light touch -- regardless of direction.
The lens is a 39mm thread screw mount that comes with an M adapter (which brings up the 50mm framelines). Bessa cameras don't autoselect framelines, so you could use an M adapter for a 135mm lens, and it wouldn't have any effect. Framelines are manually selected by a switch on the top deck.
You can use the Sonnar with any Leica screw mount camera by removing the M adapter, although you will need a separate viewfinder for accurate framing (or you can simply make your best guess). Because I use it on the Zeiss Ikon, I decided to buy an adapter that would bring up the 35mm framelines, which is closer to 40 than 50, and it's worked quite well.
Because of the physical limits of the Bessa rangefinder design, the lens decouples from the rangefinder at around 0.9 meters, although its close focus distance is 0.7 meters. This means that the secondary image in the rangefinder stops moving once you pass 0.9 meters.
Mounted on the Zeiss Ikon, it focuses accurately all the way to 0.7 meters.
As far as sharpness, I've never had any complaints.
When it comes to so-called character, signature and bokeh, that seems to be a very personal thing, especially among Zeiss and Leica devotees (and moreso with Leica users than Zeiss users). It's difficult to answer the "which is better" or "how does it compare" questions, because this is where objectivity goes out the window and people's opinions and "feelings" take over.
You probably can't go wrong with either the Sonnar or the Leica lens. The one advantage with the Sonnar is that it's new. But if you prefer the Leica look, then you should consider the Leica lens.
I love the Sonnar with the Zeiss Ikon and use it as the primary lens on that camera, rather than the 50mm Planar. The Sonnar is compact, quick to use and is a very nice lens.