The 500 IMO will provide more of a true trappist characteristic. Personally my go-to Belgian yeast from WL is the WLP 530, personally, and again IMO, 530 provides a great Belgiany characteristic that is unmatched it is the Westmalle strain, and it is delectable.
So in summary, stick with the 500 if you can. If you are bent on using the 550, try it alone, and taste the difference, perhaps you'll like it. I wouldn't blend it, unless you really want to experiment, and have a good idea of the separate characteristics of each that you will be able to distinguish between.
Next Belgian, give the 530 a go, you will not be disappointed, I would suggest a Belgian Quad perhaps.
Good luck!
Cheers!