In addition to the points above, additional points to consider could be:
Will the school allow you to sit in on a student briefing/debriefing and back seat during an exercise? This may assist in allowing you to determine whether the teaching style is something you will be comfortable with.
Be honest with yourself. If you have minimal twin time, do you think that undertaking a course that consist of 35 hours in the sim and 15 hours in the aircraft will be sufficient? Or, should you go for a course that has less sim hours and more time in the aircraft. This may cost more but allows you to be realistic about your budget.
How are you charged? Most schools will charge on airborne plus x minutes. There are, at least 2 schools that I visited, that charge on the hobbs/tacho meter which means that you can end up paying very dearly for delays whilst held on the ground.
Does the course include approach and landing fees or are they charged separately? This can amount to a few hundred pounds so it best to be aware of this when comparing the costs from different course providers some of whom will include the costs and others that won't.
Is the cost of the 170A test included or is it charged separately? If not, how much will it cost?
If training at an airfield that is not a test centre, can the school arrange for the examiner to come to your home field if more than one student is taking his test on the same day? This could save on positioning costs.
If you choose a course 'package', what are the costs for any additional hours should you need additional training above the minimums?
If staying away from home, how much will accomodation and travel costs amount to? These costs when factored in to your decision may lead you to selecting a course at an airfield closer to your home even though it appears to be more expensive initially.
Good Luck.