A lot depends on the longarmer. Not all longarm machines have a basting stitch (and not having a basting stitch makes basting really tedious and time-consuming). I don't know about now, but about ten years ago local longarmers charged $50 to $75 to baste a quilt. Some longarmers will even baste with water-soluble thread for you if that's what you want.
In general, I guess I would opt for a grid 4" apart. A frame-basted quilting grid is going to be more stable than non-frame pinning.
Traditionally you would want to baste a quilt that is to be hand quilted with basting thread, not pins, as pins interfere with hooping. Pins are preferred over thread basting for quilts that will be machine quilted, though, because you don't want basting threads caught in the machine stitches.