I work in a hospital and although i dont work in the Gastro department i still deal with Pred, Aza and other drugs such as methotrexate and mycophenolate.
Pred is def used for short term. They actually call 6mp/aza etc steroid sparing drugs.
Dermatology (where i work) has been using these drugs for a good number of years and the only long term side effects known are all well known already Basel cell and squamous cell carcinomas (skin cancers) etc. I think the doses IBD patients are on are significantly lower than the transplant patients who develop these in the long term.
There are a good number of treatments to TRY and prevent these skin cancers developing which are being looked at. there is a study called "rescue study" and this is currently being undertaken in my hospital regarding the long term skin risks of the Azathioprine and mycophenolate drugs.
I think some of drugs are called Aldara and imiquimod but dont hold me to that. (I only work in admin !!)
The new biological drugs used such as humira, infiliximab (dont know if you have that in Amercia, im from the UK) are really really new and not a lot is known about them at all.
Me personally, im happier using Azathioprine, because if you take the right precautions such as sun block and obviously the blood tests to look for lymphoma etc then i think this drug is a lot safer than pred in the long term.
sorry for the ultra long post. and i hope it all made sense.