The Forerunner 50 is much cheaper than the RS800, that's for sure! It is being aimed primarily at the RS200 end of the market but its features are those of the RS400. With bits it will retail for less than the RS200 + pod.
The F50 pod uses the same sensor technology as the S3 pod. (Garmin own the manufacturers that make both).
I run with a 305 and an RS800. The RS800 has to be quite badly calibrated for there to be much of a discrepancy between the two in terms of total distance. Even during very unevenly paced runs on undulating terrain my units come up with the same results to within a few tens of metres, and that is at distances anywhere from 6.3 to 23 kilometres. But this is with one proviso: that I avoid certain stretches of our forest trails and a single stretch of road where the Garmin loses the plot. I know where these are, so when the Polar tells me I have done 1200 metres and the Garmin tells me 1k, I know to believe the Polar (these stretches have been measured for official race purposes, and the road is marked with posts every 100 metres and kilometre stones, done by the highway authorities).
Polar are releasing a GPS module for the RS800, and there is a footpod for the 305. so I suppose it would be possible to use both technologies. But how would the software know the correct reading?
Re pace: the S3 is very responsive and returns a realistic pace within seconds of starting or adjusting pace. So the F50 will probably do the same. The 305 takes a while to get into its stride but once it does so, I find current lap pace or average pace to be pretty well accurate; I've tried this on a track and the 305 coped during 200 metre reps (after I'd done the first 50 metres!)
But ... really and truly, the data we get from these gadgets is only of historical interest. I love it because I get twice as much out of each run: the run itself and then hours spent at the computer scrutinizing the pretty graphs (Polar) and cursing the rudimentary Garmin TC software and blessing SpotTracks 
They tell us how far we've run. And how slowly. But in the end we can't run faster than we are capable of on the day, and on training runs it's surely better to listen to our bodies than to force tire dlegs along just to attain some notional target and risk over-reaching or injury in the process?