For tools, Bronze last longer than steel. But I wouldn't convert ALL of your steel into bronze, because most of the other things that can be built in minetest require steel, such as rails, buckets, carts, etc.
You can learn a lot about which tools last the longest by looking on the wiki:
http://wiki.minetest.net/PickaxeOnce you get deep enough to start finding diamond, you hardly use bronze at all anymore. I think we need more recipes that use copper and bronze.