Looking to set up a shakey head Med / XF  spinning rod , 2500 series Daiwa  outfit with either #10 lb. or #15 lb.  Power Pro braid as a mainline + Sunline Sniper #7 lb. FC leader . *Between the #10 lb. & #15 lb. braid as a main line - what would be your size choice for a shakey head  application and why ? ... Thanks in advance !

Since you are using a leader I would go with the 15 pound.

I have used both. With the leader I see no benefit to using lighter main line.

10 pound braid gets wind knots easier than 15 pound.

I was leaning towards #15 lb. braid main line  for the shakey head set up ... I may try #10 lb. for a ML Ned rig but as you stated , I fear diminishing returns on the lighter braid sizes due to wind knots increasing .