And since he didn't confine me to 20 words or less, here's my unabridged answer. Every Smith Press is designed and structured differently. The bar plus mechanics weigh about the same as an Olympic bar. Some manufacturers feature a counter-balance to neutralize or diminish the resistance of the unloaded bar. I personally don't like these interferences, and quickly dismantled them.
Some machines glide smoothly, some bind.
Another thing - the guided movement provided by the machine alters the natural bench pressing arc, thereby hampering the recruited muscles full exertion. 100 pounds on the bench press seems like 150 on the Smith. The advantages in the machine are in the control, the rigid limiting mechanics that protect injured rehabbing shoulders, chest or tri's. Neat gadget for partial pumping movements, isolation and improvised strictness. Great for front squats and split leg squats.