Smith Machines are great for workin' out by yourself . You can drop the weight whenever you need to and the machine will catch it for ya . Money well spent ...I love mine !
Actually from my experience I think dumbbells are the best. for the following reasons:
1) Machines restrict your movement.. everyone’s body’s are different and what might be a comfortable and productive angle for one person might not be right for another
2) With a dumbbell:
a) While doing single dumbbell curls, once the muscles gets fatigued, you can always use your other hand to assist in bringing the weight up. Kind of like your own personal spotting partner to help push the muscle past the fatigued stage
b) When using a machine or even a bar for bench pressing, you are limited somewhat to either a specific grip or a specific movement. With dumbbell presses, you can go out as wide as you want.. as close as you want. Start out wide and press up close. Start out wide, press up close then at the top allow the weight to fall backward a little and bring it back up. The wider the grip on a bar the more fuller your chest will get without having to do a lot of different types of presses. You do a close grip straight up and down press with a bar, then you need to add incline and decline presses to your routine to build the top and bottom of your pectoral muscles
c) Using dumbbells for fly’s is easy, and again, a dumbbell is easy to adjust for your specific body type (i.e. short arms, long arms, injuries, etc) where as with a machine you are somewhat stuck with a specific movement
d) Dumbbells are easy to push away from you if the weight ever becomes to much without getting hurt. For example. If you are pressing a really heavy weight using s straight bar. If you don’t have a partner and get into trouble.. you can become dead real quick. With Dumbbells, you can push yourself as hard as you want knowing worse case scenario you just drop the weights to the side. Might damage the floor, but at least it wont crush you.. Unless of course the dumbbells are directly above your head and you just let them drop straight down.. which I have never seen or heard of happening..
e) With a straight bar or a machine.. sometimes it is difficult to tell that one side might be weaker than the other side.. When using dumbbells, if you are doing presses or curling, it is easy to determine a weaker side and focus and building both side equally. This happened with my son 2 days ago when we were doing arms. He noticed using the same weight, he could do more reps with his right than his left. But it was not as noticeable using a straight bar for curls because your stronger side will often compensate for the weaker side and sometimes it is hard to tell on a straight bar
Machines are nice for variety, kind of mix things up some times, cables are could for certain exercises, and if you have a partner, the good ol’ straight bar bench press is nice to use to mix things up.. but for me.. most of my work outs these days are done with dumbbells. Unless I have a spotter and I want to go real heavy. Then I will do a standard straight bar press