They are much more visible than Invisalign. They are also larger / more bulky and you are not able to remove them if you need to - for example if you are going out for a meal. They give your Clinician fewer options in your treatment than Invisalign, as Invisalign have highly sophisticated software on which various treatment options can be simulated before the best treatment for you can be chosen.
Invisalign have a much longer track record and evidence to support their system, they also have a new system of retainers, much thinner and more comfortable than anything else on the market exclusively for patients who have had Invisalign treatment. 6month smiles claim to be faster than Invisalign, however in my hands, Invisalign certainly seems to have speeded up with the changes they have introduced- and a lot of great results can be achieved in well under a year, depending on the complexity of the case. Have a look at our website linked below, the before after shots were both recent cases treated in six months and seven months respectively. When I have asked colleagues why they use 6 month smiles the universal answer seems to be that they are cheaper for the dentist. You should however see someone who can advise you whether your teeth are treatable with either of these methods since they both have their limitations, as indeed do traditional fixed wire braces.
At our clinic we have had some fabulous fast results using Inman Aligners. These can move sets of up to 6 teeth effectively and in a shorter timescale than invisalign. see the youube linke for more info.