The jpeg compression from posting to the site is messing up the type a bit, but maybe that's a good test. (I've had trouble with that here. Sometimes posting a gif works better for graphics and text.)
The helvetica has a bigger x-height and the myriad is a little more open. If you are talking about 13/15, either should read fine. I'd also consider the tone of the type for the particular use.
Just curious, does the Sun Journal editorial still use vectora for its sans-serif?