@raghav--the key difference is that the 655 is a true laser, with a long history of similar units behind it. The 633 is an LED, and these "super" LED's only came into existence around 2000 or a little later; they do much of what lasers do, but are not totally monochromatic--a 655 laser emits all of its photons at exactly that wavelength, while an LED 633 has some spillover--most of its energy is emitted at or just around 633nm, but there is some spillover to the adjacent wavelengths. A laser also emits coherent light--the photons march in step--while an LED does not. In practice it seems that LEDs can do much of what lasers do, but for some purposes there is still a difference.
There is a diffusing tiny lens in front of the 655, as there is for the 633, but one still has to be careful in using it--it might do damage to one's eyes if one shone it directly at them for some time, though due to the low power and diffusion the danger seems very low indeed, and I don't think anyone has actually suffered real damage. The 633 is a shot at doing much the same work with total safety and lower cost. I find it a good little unit, but have never tried the 655, though I suspect it does do somethings a little better than the 633--more penetration, perhaps a more powerful effect on the blood--I don't really know for sure.
I find that both the 633 and "regular" 810 help me sleep, and neither seems to induce fatigue in me, but we are all a little different. Part of the trick of timing while using these devices seems to be finding the right balance between triggering the cytochrome molecules in the mitochondrial electron chain that produces ATP, the energy molecule, which then, among other things, triggers the release of some ROS (reactive oxygen species). My impression is that this is one of the advantages of pulsed units, like the 810 and the Neuro, but I really don't understand the processes involved in enough detail to be clear about this.
I have been having sessions in the morning using the Neuro intranasal 810 and my 633 on my ears, and tinnitus does seem to be reducing--will wait a while before attempting to check whether high frequency hearing is also improving.... and this makes timing of my use of the full Neuro a bit more difficult....