There is no definitive answer, but which is best for you depends on your own specific requirements. If you are having a camera converted to IR with an internal filter, which wavelength to go for depends on whether you like the false colour effects, where foliage looks yellowy gold and skies look blue. This is not an effect that can be seen in the viewfinder or on the raw or jpg file. It is obtained by swapping the Red and Blue Channels in Photoshop, see the tutorials page for more details.
The example shown on the left is a typical example of the "Goldie" look, certainly interesting, but some people love it and some hate it. You can only get that sort of result with a deep red filter, that allows some Visible Red as well as IR light through, something in the region of 565nm to 680nm filter, the photo on the left was taken with a 590nm converted Sony Nex 6. A 720nm filter, the most popular IR filter, would only have very muted colours even after swapping channels. With wavelengths longer than about 700nm, there is very little colour information, the R,G and B channels are all practically the same, so these are more suited to Black and White effects. The longer the wavelength of filter though the greater the contrast between blue skies and foliage, so a more dramatic Black and White effect is obtained with something like 850nm filter, although exposures will have to be longer as these filters are blocking all visible light and quite a lot of Near Infra Red that the camera is sensitive to.
The simple answer is if you want to do Black and White only, then go for the 720nm or 850nm, if you want to keep the option to do False Colour then go for a 590nm or 680nm. The 590nm and 680nm conversions can do Black and White, with very slightly less contrast as they stand.
If you have a 590nm conversion, for example, you can always add an external R72 IR filter to the lens to get the same effect as an internal 720nm conversion, with very slightly longer exposures, approx 1 stop at most. Likewise if you have a 720nm conversion you can add an 850nm filter to the lens to get slightly more contrast in landscapes, you cannot however put a 590nm or deep red filter on and get false colour effects. It won't work that way round as the internal 720nm filter will block any red light that the red lens filter is letting through!
In Full Spectrum Conversions, the internal UV/IR Cut Filter is completely removed, whereas in a Specific Filter Conversion [ IR Conversion, normally ] the UV/Cut Filter is replaced by a filter that blocks most or all UV and Visible light, such as a 590nm or 720nm filter. In the Full Spectrum Conversion a similar filter has to be placed in front of the lens in use, such as a Deep Red or R72 Ir Filter. The results are the same if a similar filter is used, so what are the Pros and Cons of each type of conversion and each wavelength.
Most Versatile as you can choose to shoot in UV, Visible or Infra Red by changing the filter on the lens.
Less glass in the camera's sensor stack, which means less Flare and less dust as there are two less surfaces, unless a Full Spectrum filter is installed, which is not necessary in most Sony mirrorless cameras.
The camera can still be used for normal photography by adding a UV/Cut filter to the lens.
You have to buy filters for each lens you use, this could mean several sizes and Wavelengths.
Can be fiddly swapping filters when using lenses for both Normal and IR Cameras.
Some lenses cannot accept front mounted filters, eg. some Fisheye lenses.
UV/Cut Filters are quite expensive and the cameras White Balance will never be quite the same as an unconverted camera.
or R25a, R60, or Deep Red External Filters
Excellent for False Colour " Goldie" and "Blue Sky" effects.
Very Good for Black and White
Can accept longer wavelength, eg. R72 Front Filter for better Black and White, then exactly as 720nm Conversion, except about 1 stop more exposure required.
False Colour work involves much more Post Processing in Photoshop. NB. You cannot swap channels in Photoshop Elements!!
Not quite as good for Black and White as longer wavelength filters, but very close!!
Very Slightly Softer than 720/850nm.
Lenses with Hot Spots, often show coloured area in middle of frame.
As Above, but a bit more Contrasty for B&W landscapes, but a bit less colourful False Colour
Excellent for B&W, probably the best wavelength compromise for B&W between Contrast and Exposure.
Most lenses work well at 720nm, Hot Spots, mostly neutral colour.
False Colour Gold foliage and Blue Sky very weak
Excellent for B&W, very dark skies and very light foliage.
No False Colour at all, hardly any difference between channels. Needs about 1-2 stops more exposure.
Some lenses look milky - eg. Loxia 21mm
False Colour not quite as good as 590nm
B&W Not quite as good as 720nm
Don't have to buy filters for all lenses to be used.
Don't need to swap filters
Camera is tied to whatever filter is installed
Can't use camera for any other light other than original filter choice.