Cost, usage, and cam choice IMO. The guys touched on most of it - 1.6 will open and close the valve faster for the same given lobe. The way the get the ratio higher is they move the pushrod cup in towards the rocker shaft centerline. So depending on the pushrod, head, and lifter you may have to clearance the pushrods. Higher ratios can require heavier springs because of the lost leverage on the valve side. In terms of cams there's not much difference between a 1.5 and 1.6 - but a cam designed for 1.7 or 1.8 will be a lot different than one for a 1.5 rocker engine.
And the biggest thing - you're probably not needing to worry about it at this point.
