Well, the difference between those is called their refractive index, it has to do with how quickly light passes through the lens material. The lower the number, the more QUICKLY light passes through it, so the visual CLARITY you will see is better. However, i am not sure that one of these lenses will really be NOTICEABLY thicker than the other. That actually depends more on the frame selection, any polishing you do on the edges of the lenses, and also the severity of your prescription.
i would say get the high index 1.67 over the 1.74, cause the clarity is actually better. But it is really hard to tell unless your prescription were listed here
hope that helps!
The main differences between these two lenses are price, thickness, weight, color options, and general appearance. People purchasing 1.67 high index lenses generally either do not have an extremely high prescription or are shopping on a budget. 1.74 high index lenses, on the other hand, are the thinnest and lightest plastic lenses currently in existence.1.74 high index lenses are thinner and lighter than 1.67 high index lenses. 1.67 high index lenses are much less costly compared to 1.74. Though both reduce the eye distortion caused by strong prescriptions (making your eyes look bigger or smaller than they are), 1.74 high index lenses accomplish this better than 1.67 high index lenses do. 1.67 high index lenses are available in more colors: both come in clear, gray tinted, and brown tinted, but only 1.67 comes in Transitions brown or gray.1.74 high index lenses are better for those looking for the thinnest lens possible; generally, people with extremely high prescriptions choose this option.1.67 high index lenses are better for those looking to save money or those who do not have extremely high prescriptions.
It is a fact that different index lenses have different thickness, and usually is a visible difference. For example, if you have (+) diopter lenses, so this type is thicker in the middle and thinner on the edge. Maybe it is harder to observe the different index thickness, but it depends on how big the frame is. I recommend larger frame for a thinner lens on the edge, so a 1.74 lens would be thinner. There is no way that two different index lenses to be the same thickness. Maybe is not a big difference like between 1.5 and 1.67, but the 1.74 is thinner for sure than 1.67.
For the price, 1.74 is more expensive, and 1.74 are more thinner than 1.67, if you have a strong prescription, you had better ordered 1.74. in addition, the brand of lenses are also important. Better brand have better purity,as well as better surface polishes.