There's a lot more than a simple "one vs the other" if you consider everything holistically. As John Yang said either you need/want the power or you don't. One nice value add of going Benchmark is that there is the no-risk free trial, but this is moot for Canadians as you will be taxed regardless at the border if you choose to return it. Similarly Benchmark also has good warranty and support, but with warranty claims, shipping heavy boxes to the US from Canada is also not cheap if there were to be a problem so you have to factor that in if you want to factor something like warranty in the decision.
It's also not a matter of paying $3000 USD for Benchmark vs. $2000 USD for the NAD. After import taxes to Ontario the Benchmark ends up costing $4250 CAD which is $3420 USD (this is not counting any potential additional handling/customs brokerage fees by a courier if Benchmark uses one). There are a few domestic sources that have the benchmark for sale but the price is identical if not more expensive, so unless the domestic vendor provides their own impeccable domestic warranty there is no advantage to buying domestic in this case. And while it is easier to get the NAD domestically it is $3164 CAD after tax (more costly than the US price after conversion) which ends up costing $2550 USD. It might be cheaper/lower cost to get NAD warranty fulfilled from various retailers or NAD directly but then you have to factor in that NAD themselves are known for having lacklustre support. It may also take many weeks or months to get your equipment returned to you from either of these companies.
If you really want a more relevant answer you need to consider what pair(s) of speakers are being driven now and in the future, listening levels, along with the price/your financial situation and how much you care about things like aesthetics, warranty etc. Remember that these specs are only at a specific output level. If max fidelity is your primary concern above all else, the Benchmark may be the objective winner at a typical 100~200 watts, but Purifi-based not only has more power but it is similar and even better in the upper portion of Benchmark's rated stereo output range and beyond, so it still plays very well as high as 250-350 W. It is a more versatile "tool" in that respect, albeit perhaps may not have the same longevity or reliability as a Benchmark product (conjecture). In the case of that amount of power, it may make more sense to compare NAD/Purifi to other amplifiers but anything Purifi based will probably be superior to other products at comparable power outputs and at almost every price point.
If money if no concern you have the option of buying multiple AHB2, but I assume it is a concern if you are considering a single AHB2 stereo vs NAD/Purifi. I think for 95-99% of people a Purifi amp is probably going to be the better choice not only for your money but for the output versatility. But this opinion is strictly assuming it's a DIY/custom Purifi amp that would cost significantly less than the NAD. At $3000 vs $2000 it's debatable, but in my opinion at the Canadian prices of $3500 vs $2500 the value of the NAD product is not retained, especially when you can put together the same/better performing amp for half that price and 30 minutes of your time (with no technical knowledge required).
Not that I'd want anything from NAD (not always dependable). 
