It's difficult for people to make comparisons as most will only have had experience of one school and if they've been successful there is bound to be a feel good factor to consider.
As for me I can only speak for BGS - I've just done and passed my first 8 exams I therefore feel very good about this school.
I have however been in education and training all of my working life (teaching kids, training adults, and publishing software) and so I can draw on other experience...
BGS Material - excellent
BGS tutors - excellent - individually and collectively they have massive experience, (been there, done it and got loads of T-shirts, as they say)
Online support - quick, efficient and extremenly useful (look for yourself at
http://www.jals.co.uk/forum/ to see what's going on. You don't need to be with BGS to register here or in fact take advantage of the BGS expertise.
Online Question Bank (extra small charge) - don't even think about this one - just register and use it till your mouse falls apart.
I can't comment on other companies; in the end you pay your money and you take your chance. They may be as good as BGS, they may not; I simply don't know.
Good luck
Az