Hey Steve...
In a street engine, iron guides will work fine and last the longest. If you go to bronze, you have to open up the valve stem clearance at least .001 to .002", as they will tend to scuff the stems if you don't, and often stick if the clearance is as tight as the manual recommends with bronze. Iron guides can be tight enough that you need no other seals than the spring retainers to limit oil usage through the guides. If you go bronze, you will use a bit more oil, and it may even be a good idea to use special intake guide seals.
As you can tell, iron guides are simple and easy. Bronze guides take a bit more planning for clearance and seals...and are usually used on very high spring loaded valve trains instead of stock style engines.
If you replace the guides, it is mandatory to re-cut the valve seats. Any time the guide is replaced, the valve stem center is altered a few thousands in random direction's. If you try to change guides and then just put the valve back in, the valve will hit crooked every time it sets down. This will wear the new guide and make the seat oval...both not good things! With new guides, you must grind or cut the new seats to center concentrically with the guides.
Hope this explains why...?!?