Good call. VS has an excellent J product, that (at least until recently) includes nightgear (pajamas) and an amenity kit that is, depending on AC's flavor of the month, generally superior in quality - though not quite the Bulgari kits you get on some others. The shoebag is ever popular with the ladies - nifty one, at that.

In flight entertainment - VPort - is top notch - with a significantly larger selection than AC's entertainment.

VS also offers free chauffeur car service to/from heathrow (or a free first class ticket on Heathrow Express) on certain fare types. Officially, they dont offer this service to discounted J ticket holders, but they often make exceptions (once on a discounted J outbound, Prem Econ inbound fare). And, of course, the LHR VS Clubhouse is a hell of a facility - massively better than AC's flagship lounges. The food selection is excellent.

The food quality on board is also excellent as is the service. On some ex-US flights, they offer an option that will allow you to go to bed straight away, undisturbed, rather than be forced through the meal service - which I find an excellent touch.

AC might offer a better transit experience through YYZ (which has arguably one of the most boring international dutry free areas in the world), but if fares are equal, VS's overall product is significantly better (unless you're collecting *A miles).