Speed accuracy at high update rates

I'm looking for a GPS for accurate dynamic speed monitoring with
limited budjet. It can be USB/RS232/Bluetooth or whatever, but I need
to get the output to my own program.

Standard GPS can output speed only at 1 Hz and even that can be a
floating average thus not the true speed of the last update.

Now there are available cheap 4 Hz update GPS models based on u-blox
Antaris4 chip and a 5 Hz model by Garmin (18 5Hz). On top of that VBOX
has rather expensive GPS models with 5, 10, 20 and 100 Hz update rate:
http://www.racelogic.co.uk/_downloads/vbox/Datasheets/Overview/VBOX_Product_Overview.pdf

What is the real different between all these? Do they all use same
method (doppler) for speed calculation? Do they all have the same
accuracy for each update? Or does the higher update rate result in a
worse speed accuracy? What kind of speed noise of individual update to
expect with all these?

Then to another slightly related matter: Are there any "normal" marine
GPS models (handheld or fixed) that support higher update rate than 1
Hz? Which models support even 1 Hz update to NMEA at 4800 baud? My old
Garmin 48 only outputs 0.5 Hz at 4800 baud (1 Hz at 9600). I know 4800
is rather slow, but I wouldn't really need that many sentences
(probably just GPRMC). According to manual Garmin GPS 18 5 Hz would be
able to send just GPRMC at 5 Hz, but it is just a "GPS mouse". Most
"normal" GPS manuals don't specify the actual output rate.

Joakim

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