L3/m is not the same as L5/M so they don't mean the same thing. L5 is 'putting them where you need them' or where a specific risk or situation requires them, e.g. an access room, but no where else.
L3 is clearly defined with detection to MoE and areas opening into them (slightly more detailed than that I know, but you get the jist)
Precedence is dependant on the risk, hence the risk assessment legislation. Based on our AFD policy we would tend towards L3 as a minimum, but it does depend on the premises, usage, etc.
Bear in mind you want the doors to close before they let significant amounts of smoke through and of course heat/fire. traditional solutions were to put DS either side, but this doesn't account for passage of cold smoke.
I'd go for L3 unless I was very convinced that the response by manual means and shaft (l4) or risk specific (l5) would be quick enough to ensure prompt closure.
Also you need to ensure if you are looking just at life or at property protection too - Not every SC/A is shut at night like it should be & if you are relying to a degree on manual response you could have problems