Check each firm on-line; they will give you coverage for where you are and 02 were honest enough to say they didn't recomend their system for where we are but 3G works fine.
I subscribed to 3 last summer & regreted it - just after I signed up they withdrew 2/2.5g connectivity, so instead of getting a slow connection outside 3G areas I got nothing. I'm now with Voda and I can't remember having significant problems on the way back along the SW coast last year (apart from up the Yealm where we were incommunicado). There were places where my O2 phone had signal and my Vodafone dongle didn't, but there may have been places where the reverse was true.
My boats in Mayflower Marina.
With a 3 dongle I really struggle to get a good connection.
Manytimes I can connect but at such a low rate as to be useless.
The 3 coverage map shows 3g and HSDPA in this area but experience shows otherwise.
Some of the effect is from all the masts in the marina.
They interfere with TV reception as well.
I subscribed to 3 and regretted it. I am in a so-called high coverage area but got sub-dial up speeds. Once you sign up for it you have no right to return the thing and you are stuck with a long contract. If you try and duck out of it they set a debt collection agency onto you.