Having used both, my clear preference is Miralax.
Miralax is easy to take as it can be added to water, juice, tea, coffee and dissolves completely. It adds absolutely no taste nor alters the texture of the drink and hence can be given to someone without being noticed. This is good for people who won't take meds. The normal daily dose just softens poo, making the normal process of evacuation easier. The dose can be adjusted to make poo firmer or softer. The effects are usually noticeable after 12 to 24 hours although you should not expect urgency or a massive cleanout. When a much larger dose is given, like 8x as in a bowel prep, dramatic results follow within a couple of hours and you poo liquid.
Colace doesn't really work well in my experience. While it somewhat softens poo, it also makes the poo very sticky and evacuations don't feel 'complete'
If more is given to further soften, I've wound up with the runs, having to poo very loosely every half hour or so.
IMO, the clear choice for maintenance is Miralax. If I get constipated, I take a dose of Miralax in the am, then a dose of Senna plus a dose of Miralax about 12 hours later. The timing and combination results in an easy soft poo. Thereafter just Miralax 'keeps things going' easily. The dosages can be adjusted up- or down to result in the softness and urgency you prefer.... more Miralax = softer poo; more Senna = stronger urge. Effect can range from a gentle hint to an impossible to resist urge.