Hi jaslang
Glad to see you may be purchasing a new Dongle.
There's no difference between the old and the new modems in terms of speed, they both offer 3.6MBS download speeds in 3G+ (HSDPA) area's
In terms of comparison against a fixed line broadband, it’s difficult to say. I've just moved My home provider to another provider, and actually find my Dongle faster than my current home service in my local area (I'm being honest and not biased, my home connection gets traffic shaped massively which has a knock on effect of not being able to stream a youtube clip.)
It all depends again on your area. Have you seen Vodafone's
Coverage Checker? This can give you an idea of coverage for your postcode.
Text commentary would count towards your usage as it’s still downloading the page as it updates.
Online poker can eat your usage relatively quickly, it depends on how graphic heavy your client is.
It may be worth comparing your usage after a session to accurately gauge how much credit this will use.