My friend, a fast reply to your answer is that IR wavelength and brands depends only in the kind of colors you want to show in your images and the durability of the filter you want. The brand doesn't matter if you will use the filter a few times per year, so go for the cheapest you could get.
But to understand the 650nm, 670nm, 720nm, 850nm, 950nm, 1000nm numbers, you need to learn first how the light work.
The next figure is the representation of the wavelengths of diferent type of lights, the human eye can "see" only the waves between 400nm and 700nm, we call them, COLORS. Above 700nm is the Infrared light, that's the light produced by heat, but human eye can't see that, only feel it as radiation. (IR is btwn 1000nm-10000nm)
Well, the IR Filter function is cut the wavelength light under the XXXnm of the filter. In other words, the IR filter let pass the wavelengths (colors) over the XXnm. So the colors under the XXXnm will not be seen for the lens, but over XXXnm does. As we can see, red colors have 700nm of wavelength and in the market there are filters from 650nm to 1000nm.
Finally, if you want to do an image that shows light red color, dark blue colors and the heat over the objets, you must buy a 650nm, 680nm or 720nm filter. If you want only the heat you must buy 850nm, 950nm or 1000nm.
In adition of all of those complicated things. As the IR filter let pass lower quantity of light, you will need to increase the exposure time (Lower Shutter Speed), increase the ISO value and decrease the f-stop (Higher aperture) when you take the image, in the way you use a higher XXXnm IR filter.
So my recomendation is get the 650nm for indoor (low light ambience) or get the 950nm for outdoor (day ligth) plus a tripod. If you want a versatile one use the 720nm.
I hope my explanation was useful for you.
All the best,
Nicolas Duarte from CHILE
the thicker, the bettermakes it sound like you're only talking about the physical dimensions of the filter; You know that the nm's are the wavelengths of the light, right?