Polarisers are one of the most useful filters for landscape photographers and sometimes they are used even on wide angle lenses.
If you are one of those photographers like me that always uses graduated filters probably you are using the Lee adapter with a 105mm ring adapter for the polariser.
These filters are really expensive and hard to find, even if really big in size if used with a wide angle lens they tend to vignette much at bigger apertures, so when on the field you must pay attention to not shot at the wider angles to not get an image that needs to get cropped later (or worst, rendering it useless when the subject is on the corners).
Due to this issue I have tried at home two of the most used filters to better understand the limits of this system.
For the tests I have used a Lee holder with 2 slots and the wide angle ring.
Camera used is a Canon 5D MarkII with a Canon EF 16-35 f/2.8 II lens.
Strangely enough I have found that the Heliopan filter, even if a little more thin than the B+W vignettes more.
Focals under 21mm are useless due to the heavy vignetting, then the effects can be easily corrected in post production.
Reducing the number of slots to only one the vignetting should be reduced but I have not tried because I find too limited an holder with only one slot.
In conclusion I have not found great differences in vignetting with the two filters, so I think that you can buy one evaluating other parameters.
For example I have to say that I find the ring of the B+W too hard to rotate, the Heliopan conversely has a smooth ring that is even graduated.
Other parameters like sealing and optical characteristics can be found on many reviews and data sheets.