For simplicity, many physicians believe in the 'class effect' of medications. So that all ARBs are created equally when it comes to blood pressure reduction and/or disease protection. This is likely false, though proving small differences among the compounds is difficult. Therefore, when it comes to over all organ protection, I don't practically distinguish between medications within this class.
Having said this, there are likely differences within this class when it comes to blood pressure reduction. Later generation ARBs (like olmesartan) are likely more potent from a BP perspective than, say, losartan (both drugs interestingly made by Merk when they acquired Schering Plough). So in my practice, I do often switch to later generation ARBs when BP is still not at target.
Lastly, I do not prescribe like prescribing generic medications when it comes to treating cardiovascular illness. Despite what some might say there is no evidence to say that generic medications are in fact similar to originator molecules when it comes to disease outcomes. Generic medications should be avoided when treating illnesses where mortality is a possible result because quite simply, the entire spectrum of generics is untested in terms of actual patient outcomes.
In order to get FDA approval to market a generic drug, it has to be proven to the FDA to be "essentially equivalent" to the branded drug. It is subject to the same level of manufacturing inspection and control as the branded drug. Check the FDA website for this. There may be some cases where due to manufacturing/compounding differences the drug will behave differently with some persons. If your doctor agrees to try you on a generic, a 30 day trial to be sure that you are getting adequate BP reduction from the new drug is in order. If it does the job and causes no other problems, I'd go for it. Losartin (Brand name Cozar) is now available as a generic and my Medicare part D provider has it on the tier 1 list (lowest copay). I think you can get it pretty inexpensively at Walmart or Walgreens. Ask your doctor!