I think the
mailto: link is a better option for mobile users.
The contact form gained prominence [original research] because on a desktop computer it is quite rare for users to have a mail client installed. Instead of having a link with a
mailto: address that would simply fail in the majority of users' browsers, people started adding a form which would duplicate the functionality. If Web Handlers become more widely used and supported, we may see this start to change, but the fact remains that a very large majority of users will click a
mailto: link and have nothing happen.
On a mobile device, however, this is entirely the opposite: Every smartphone has a mail application pre-installed on the device, capable of handling these
mailto: links with ease. This also avoids forcing the user to wrestle with their device's browser's text box controls, instead using the relatively mature controls of their e-mail application.
In short, use
mailto: for mobile, because e-mail apps have better controls.