I have only studied with one, so my opinion can't be fair.
Bristol provide a straight to the point ground course, their books are easier to study than Oxford's (have used some of their books), but less detailed!!
The 2 weeks crammer course, before the written, are provided by top notch instructors (ex military and airline pilots) and most students are surprised by how they are ready when the exam come.
And last but not the least, their ATP online database is the best tool to finish preparing you for the exam and most atpl groundschool in the UK invite their students to use it, or that's what i was told by such students during the exam at Gatwick.
One free advice would be to get the Oxford Meteorology CD (great tool) whatever your choise.
Good luck in your study and good flight.