With point and shoots the depth of field differences won't come into play too much except for maybe some macro shooting. Your main concern is how the wider apperture affects the exposure, allowing more light in.
For exposure purposes, if you multiply the f stop by 1.41, that cuts the light in half (a one stop difference). The difference between f2.8 and f3.35 would be a half stop. From f4.2 to f5.9 would be a full stop.
For low light performance, you really want to look at four factors:
1. How bright the lens (max apperture)
2. High ISO capabiltiy
3. Mechanical (or optical) image stabilization
4. Flash capability
Image stabilization can often be good for 2 stops, but keep in mind that it doesn't help with freezing subject motion. If you are worried about performance at the long end of these zooms, though, without IS you might normally need a shutter speed of 1/60-1/100 sec to prevent blur from camera shake. If you are dealing with shots with little subject motion, you might be able to instead get shots at 1/20-1/30 with IS.
High ISO ability will let you shoot at a faster shutter without losing light. If you need that faster shutter anyway to prevent subject blur, than your only choice may be to boost ISO--IS won't help much here. Keep in mind that using a higher ISO also extends your flash range. A flash that is only good for 5 feet at ISO 100 may be good for 20 feet at ISO 400. With a usable ISO 400, you may find that you can use the flash effectively even using the tele end of these lenses (which are only in the 100-115mm equivalent range).
While it would be ideal to have a compact camera that is a top performer in all four of the above, I don't think such a camera exists. The cameras listed may be as good as you will get. The Fuji Super CCD models are about a stop better than the Canon at high ISO (compare ISO 400 to 200, and 800 to 400 in the link from rinnie above); but the have no IS. I think there was a Panasonic model with IS and a faster lens, but noisy even at ISO 200.
From the above models, I would tend to lean towards the SD800IS, but that's in part because it has the 28mm wide angle. If you really don't want the good wide angle, you might also look at the SD700.