I am currently in the process of setting up a site with ProcessWire and pretty much the only alternatives for me were ExpressionEngine and WordPress. WordPress just isn't built for anything but blog posts (I am aware of the 'custom post types' but it's a real pain requiring tons of hacks) and while I used to be a fan of ExpressionEngine it's just not a good fit if you don't have $ to spend. There are excellent add-ons for EE but I'd need quite a couple of them to get the same functionality that PW provides (Playa versus multiple related pages/PageArray in PW, Matrix versus Repeaters in PW). Additionally the limitations of the free version (e.g. the discussion forums being missing) and the license fee kicking in as soon as I try to monetize a site in any way kind of make me hesitate building the kind of "Let's see how this works out" projects that I often mess with on EE.
I am not sure whether the concept of EE served as an inspiration or blueprint for PW, as they are really quite similar but as others have pointed out in this thread, the whole transition should be super-easy as a lot of the site building blocks are the same. Even if you didn't intend to do so (and I don't think so at all), you have created quite the 'ExpressionEngine killer' here... and eventually the word will spread.
That said, ExpressionEngine is a great product and I hope EE (the product), the community and EllisLab keep on going strong. And if you can afford it and your customer feels better about using a 'commercial' product backed and supported by a company, definitely go for it. From what I have seen so far the EE backend still seems more 'pro' and if you use the right add-ons, a couple of parts are more convenient and polished. (That won't stay like that forever though.)