The battle between free weights and cable machines has gone back and forth, each side claiming it's own benefits and drawbacks. The regular bench press is a free weight exercise. The barbell can travel wherever it wants to because it's not connected by anything. The Smith machine is more along the lines of a cable machine because the motion of the bar is fixed.
Smith machines have a barbell that is attached to the machine and moves up and down in a straight line. A regular barbell would not only move straight up and down, but forwards and backwards, which is the more natural movement found in the bench press.
Not only is the path of the bar different in both exercises, the weight of the bar and the safety of each exercise is different. Overall, the Smith machine bench press is easier and better suited to beginners.
While a regular bench press allows you to move however you want, it is also slightly more dangerous than a Smith machine. On the sides of a Smith machine are hooks that you can slide the barbell into. If you're struggling during a repetition and you think you're going to fail, you can simply lock the bar in place and escape unharmed. In a regular bench press you need to have a spotter if you are attempting a weight of which you are unsure.
Difference in Technique
Regular bench press form goes in two directions. At the top of the movement your arms are straight and the bar is directly over your shoulders. As you lower the bar to your chest the bar also moves down towards your feet a few inches. At the bottom of the movement the bar should touch the lower half of your chest. From there you press up and back until the bar is back over your shoulders.
On the Smith machine you can't move the bar forwards and backwards. You need to position your body so that the bar will be in the correct position -- able to touch the lower half of your chest -- at the bottom of the lift. You also lose some of the power of the move when the bar move on a fixed plane in the Smith pres. The bench press motion of pressing back and up actually gives you momentum and allows you to use your shoulder and chest muscles a little more.
The freedom of motion in the regular bench press also means that you need more control, so you can't press the weight up with reckless abandon like you can with the Smith machine. While you may think that the added stability of the Smith machine would help you press more weight, that's not necessarily the case.
A 2010 study in the Journal of Strength and Conditioning Research explains that the unnatural bar path that the Smith machine forces you to use actually makes the movement less powerful. The added stability of the Smith machine is essentially canceled out by the unnatural bar path and you end up being about equally as strong in either movement.
Difference in Barbell Weight
Another difference between the two forms of bench press is the weight of the bar. A standard bench press barbell weighs 45 pounds. However, in the Smith machine, the weight is often lighter. The Smith machine is designed to make the bar glide up and down and the apparatus on the sides of the machine that allows it to slide also can make the bar lighter. Many Smith machines reduce the bar's weight by 10, 15 or 20 pounds.
If you're worried about how much weight you can use on the Smith machine compared to the regular bench press, you have to make sure that you factor in the starting weight of the bar. If it isn't clear you can contact the manufacturer or compare it to a regular barbell. Make sure you check the weight before you brag to your friends!
Different Muscle Activation
The Smith machine and barbell bench press have the same muscle activation for the pectoralis major, biceps, anterior deltoids and triceps, according to a 2016 study in the Journal of Exercise Physiology. However, one shoulder muscle is differently activated in the barbell and Smith machine.
The same 2010 study in the Journal of Strength and Conditioning Research mentioned earlier concludes that the regular bench press uses the medial deltoid much more than the Smith machine press. The researchers concluded that because the regular bench press works the same muscles as the Smith machine press plus more of your shoulder muscle, it's the better choice for upper-body muscle development.