I'm taking 6-MP since almost two months and the temporary side effects from it are nothing compared to being on Pred for just a few days. In the beginning I had daily headaches (tolerable, but annoying), slight nausea (eating Altoids helped me a lot), itching and tiredness. I am still tired after taking the pill (I take it at 6 PM) for the rest of the day, but it really helps me sleep. Other than that, I went to weekly blood tests for the first month of taking 6-MP and now I go biweekly and so far, everything is okay.
I started 6-MP while on Pred, because I had a bad flare (lasted 5 months and it is yucky heyday, I had 20+ bloody BM) and I am almost free of symptoms for at least a month. It might be conincidence or it might be that the 6-MP worked extremely fast in my case. So far, I don't regret going on 6-MP (and I was very scared myself) and if it does the trick, I would like to stay on it.
Just follow the instructions of your doctor and have your bloodwork done religiously, don't miss appointments and your doc will be able to take you off 6-MP early enough before something bad happens. But the decision is yours - after being so reluctant to take it, I wish I had started it sooner.