open D and open G for delta type slide playing - G (or A) is more country delta-ish
sounding to me while D (or E) is more Chicago-ish ala" Elmore James...although Robert Johnson, Son House & others. used open D for some of their stuff
On acoustic sometimes I like the drone of dropped D tuning
I tune down a full step (d,g,c,f,a,d) on solo acoustic so my E chord is actually a D, A is a G...etc that fits my vocals better - D sounds a little darker to me also