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.