As with anything that depends on a variable service the best device for your needs will depend on the 3G signal, in the area you
will use it.
The technology offered by all the operators is the same so the maximum will be the same - the actual signal available is the important thing
The best way to judge this is by checking with actual users, try any friends/family/colleagues who use the service from the different operators, this can be from mobile phones but remember that the data and phone service are different so mare sure you check data availability.
As for which tarrif/deal, all the operators have multiple options available choose one that suits your usage and only after you have this look at free extras