The 3 dongle speed can cope with 21mbps, I think it said, but it will depend on the signal strength in your area. The only way I can think to check the speed is on a data speed measurer available on line, which is no good if you haven't bought one yet. I got a free sim card from 3, and found I could get 3 bars of 3G, with our current Vodafone, we get 1 sometimes 2 bars in the garden, no bars inside.
Regarding price, it will depend on how much data and phone use you use. I use maximum of 300 mins per month and just under 2mb data on the netbook, so for us having seperates is cheaper. £10 phone + PAYG 3 data = less than £25 per month.
Some companies unlimited data, has a fair useage policy, meaning you are restricted, the all you can eat data is 3's name for it, but theirs is not restricted.