The same material with a smoother finish will last longer under cyclic fatigue conditions.
If you were to try to pull the steering arms apart, they would break at a similar strength, but if you were to load and unload them day in and day out for years, the one with the smoother finish would last significantly longer. The main difference comes from the idea that it's easier to start a crack in a material that already has a lot of surface flaws ie. a rougher finish.
That said, you should buy the forged arms and polish them yourself, or just go nuts and shell out the extra cash for the ready-made billet.
...the end.