Brussels has some narrow streets where wide angle will be of more use than tele. OTOH, a few of the buidlings (city hall, cathedral) might have some details you might want to get a close-up shot of? Depends on your taste...
Personally I tend to shoot more wide than tele in a city. My most used lenses around Brussels would be the DA15, DA21 and FA43.
I also have the Sig 8-16, but generally leave it at home when I know I'll be walking around in a city. Since I want things in a small bag in a city, it's often too big to fit (plus too heavy (I feel it needs a grip to balance out), and it may also attract too much attention), but I can imagine it can result in some really nice shots. More so than tele. A 40mm lens could be a nice (and compact) addition to complete the kit (I rarely need anything longer), though I would miss something like a 21mm or 24mm for more general purpose shots.
Of course, If you're into architectural details or "sneaky" (
) people shots, you need longer than 40mm, but the 135mm should be more than long enough.
hth, Wim