Hi DarrenX,
You've gotten some very good feedback here. In sum:
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Let your customers review you where they want to. A diverse review profile is good insurance against massive review loss (which happens in big waves on Google). If you lose some reviews at one site, at least you still have some at other sites.
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When you have a chance to gently steer a customer toward a review site, Google+ Local is always going to be a top choice, because they dominate all local business verticals. Yelp is the obvious runner-up, given how much prominence Google currently gives their pages, but it is against Yelp's policies to ever ask for reviews, so you have to be careful there. Then, depending on your industry and geography, as members have said, you will find that certain directories happen to attract more reviews than others. See where your competitors are prominent and keep those directories in-mind.
The above is best practice advice for local businesses. However, you indicate that your current client is virtual rather than local, and develops software. I am not experienced with your industry, but would suspect that product review and technical publications/websites would be your target review sources. Maybe things like: http://reviews.cnet.com/?
I think the key here for you is to figure out where your customers are. Where do people go to find software reviews? That's where you need to be prominent.