how not to burn bridges between two academic jobs? Last year I have been submitting my CV to different faculties for a lecturing position. Of all the ones that I applied only one was more serious and wanted to keep in touch (institution A), the other one (institution B) was completely silence and only they reply with a general "we are interested, but we will tell you when there are open positions". Actually the following has happened:


Institution A has offered me a full time position with good salary, and the freedom to make research or even to work in other places part time.
Institution B contact me a few days before the offer of institution A, telling me that they would like me to work part time with them; when I told them that I would like to have a full time position they started to tell me "that procedure is going to take time". A couple of days passed and I got the offer from the job that I mentioned in the first point.
Now last week Institution B called me and they told me that they want not only to offer me a full time position, but also a position as a student coordinator of the career. The drawbacks are: the salary is 14% less than A, the hours are not; fixed that means that one should stay for overtime unpaid if the goals have not been reached, the research is keep to the minimum and I will not have the chance to work in another place part time. The only benefit is that I will be have a higher rank than in A. This same institution has already booked me for some partial hour lecturers in a couple of courses. I have been sincere with them when they called me and I told them that I have been offered another position (from institution A), so I need to wait to their answer. Even with that institution B has continue insisting me for getting the job with them.


So by factors of time, research and money; institution A seems a pretty good choice, but how I can reject politely the offer from B without being "marked" for not working there or not to burn bridges with them. Maybe somebody could say "why I do not want to burn bridges with them?, well it is just to have like a backup plan. Also institution B has the story that they are used to cut off their lecturers at any time they want, so what to do?

Thanks
What stood out to me is:


  This same institution has already booked me for some partial hour lecturers in a couple of courses.


Have you, in any time, either written or orally, communicated with institute B indicating that you will take up the position (even just the part time one)? If so, the polite thing would be to honor this commitment with Institute B, even just for one term (if you really want to work at Institute A, you can try to talk to them to see if they can arrange for the offer to be deferred for a year or a term). 

If you have not indicated to institute B that you would take up the position, and they, without your knowledge, advertised you as a part-time lecturer for their courses, I would be very, very wary of joining their faculty. It is simply not the case that a department can strong-arm a potential hire into working there just by listing his or her name on a website. 



Aside from the above: the job market is competitive. It is expected that you will be looking out for your best interest. As long as you have not made any formal commitment to either of the institutes, you are free to choose which one to affiliate yourself with in the future. If you really prefer the offer given by Institute A, you should just be honest and reply to Institute B and say that Institute A has given you a better offer and you regret that you will not be able to work for Institute B. 

This really should not be a situation that counts as "burning bridges". From your description it sounds like just the normal competitive hiring process. If the administrators at Institute B are the type that will hold your declining of their offer against you in the future, I really doubt you will want to work for them anyway. (That just doesn't sound like a healthy work environment.)