It's a matter of tolerance of the protocol, convenience of when you would like your BM to be.
I did both.
The Salt Soln (ss) took about 45min -1hr to kick in and then another 45 min-1hr of must be near bathroom. For me, I needed to be no further than the other room from the toilet.
The Senna Tea (st) I had to try several times to get a ballpark idea of how long it would take for me to go. Finally, 11 hours, so take around 7-8pm for 7am BM.
This works differently on different people you need to figure out how it works on you and how your body responds. My sister started out with st on Friday, 5 hours and 11 hours, on the first 2 consecutive days, her body is adjusting and will fall into a routine with the tea.
I would recommend you start with a transition day or two, say thur or fri with reduced food intake (25%) start lemonade drink, and laxative/flush routine. Next day, start proper Master-Cleanse
protocol on the Fri or Sat so you have the weekend to fall into regular Master-Cleanse