Is 'R' in impedance equation an AC or DC resistance?
It's the real part of the impedance at whatever frequency you're calculating the impedance for.

Very often, the \$X\$ component will be proportional to frequency so that in the limit as \$\omega\to 0\$, the impedance is \$R\$ making \$R\$ the DC resistance. 

In other cases, \$R\$ also has some frequency dependence. Then the DC resistance is whatever value \$R\$ has when the frequency is 0.