Streptozotocin causes partial destruction of beta cells. In my opinion, streptozotocin induced damage is severe than hyperglycemia induced in diabetic rats. STZ affects different metabolic pathways including DNA damage.

Dear Tarek

It is not an easy question. In my experience, damage caused by STZ is greater and not reversible when hyperglycemia is treated. Probably the main way to differentiate would be the reversibility.

Streptozotocin induced damage is severe than hyperglycemia induced in diabetic rats.

yes agree, Streptozotocin-induced damaged can not  be reversed but hyperglycemia-induced damage can be reversed. For hyperglycemia-induced effect see the paper 

Neuroscience. 2013 Mar 1;232:64-73. doi: 10.1016/j.neuroscience.2012.12.011 .

http://www.sciencedirect.com/science/article/pii/S0006291X1401643X

http://www.sciencedirect.com/science/article/pii/S0306452215007150

STZ , as stated by several people above, is toxic primarily to the beta cells and does not produce significant toxicity to other organs. Thus the toxic effects of hyperglycemia and hyperlipidemia are due to a lack of insulin. If you treat animals with insulin or other agents (metformin etc) you can reduce the toxicity of the STZ induced diabetic state.