In looking at any competing camera models, comparing like for like is the best bet. Let's start with focal length.
- Ricoh: 28mm (35mm equivalent)
- Sony: 24mm - 70mm (35mm equivalent)
This is actually a big point as these are fixed lens cameras. You have none.nada.zero ability to zoom from the camera with the Ricoh. You can "zoom with your feet" or crop the image to "get closer" but not with the camera.
Next, let's look at the IS spec. Before getting into this, keep in mind that hand-holdable shutter speeds are usually 1/focal length. That's the "safe" range...many shooters with steady hands can go under this. So, for the Ricoh, your slowest shutter speed should be 1/28 second for you to hand hold the shots. And you're right, the larger sensor size will award better image quality in low light. To me, this effectively negates the need for IS in 99% of the situations that you'll be shooting in.
Next let's look at the apertures, f/2.8 vs f/1.8. This is a one and a third stop difference - not huge in the grand scheme of things, especially since the Ricoh will have better high ISO performance.
Recap: At 28mm, the Ricoh is more functional. The larger sensor will allow for higher ISO's at the same image quality such that any benefit of IS or 1.33 stops will be minimal, if not negated.
However...The Sony zooms. Full stop. These are not like-for-like options. If I had to shoot something at 28mm, I'd grab the Ricoh. If I wanted to shoot something at 70mm...the Sony is the only option.
I would urge you to strongly consider this factor. Even in the world of DSLR's and Mirrorless ILC's, many, many people use zooms over fixed focal length lenses due to the trade-off's.