HIX is a relatively new brand with the majority of properties in renovated or newly built structures. There is a consistency to the product and thus one is likely to be less "surprised" when opening the room's door. This is not the case with HIs which, as someone noted, is a classic brand and thus has a major variance among properties. Properties are older and many still have not been brought up to a new standard.
Also, HIXs represent better value given the night rates can be up to 30% lower than an HI in the same market. Throw in the breakfast, and pretty much all one's needs are met.
But location is still the deciding factor for me. In Edmonton, a city I visit frequently on business, a newly renovated HIX opened up in the city centre just at the time I needed something more cost-effective than the CP, Westin or then-Sheraton. The HIX was perfect from all standpoints in meeting the need.
I will opt for HIs and CPs when I am familiar with the properties and I know I will be able to benefit from my Plat status for upgrading and other amenities. But when budget is the issue, HIXs tend to get my business -- as do Hilton Garden Inns and Sheraton Four Pts.