There is a slight difference in the coating I believe but it releases the contents at the same PH level. Competing drugs have come out with higher dosage strengths per pill. Therapies are also using higher dosages for inducing remission.
Asacol is going to lose it's patent protection soon and people like taking fewer pills. Simply put...its for the money.
Currently the price for Asacol and Asacol HD is about
the same for the same dosage. That will change(I hope)
A clinical study came out (I believe funded by Lialda's maker) showing that taking split dosages isn't necessary to maintain remission. This
opened the flood gate for a crop of new purple pills. The multiple dosing probably originates with sulfazine drugs which have very common adverse reactions which increase with dosage. Mesalimines have much fewer problems and higher dosages don't seem to increase them.
The difference between mesalimine drugs is the delivery method. Probably just a convenience thing with Asacol and Asacol HD. The drug companies claim that less pills means higher compliance which can outweigh some amount of less effectiveness. If it doesn't bother you to take a bunch of pills, take the one that costs you less.
I am taking Asacol HD because my GI doctor runs clinical trials from his office IMHO. I also stretch the monthly dosage 3 months and save the copay. Don't tell him or my insurance company.