Yes, there is a significant difference in tone and headroom. EL34's are more British sounding (think Marshall mids) and 6L6 are more Fender style with more headroom and low end than EL34's. This difference will obviously be more noticeable the louder you crank the output knob (to get the power tube saturation in the mix).
I have a RK1 and my favorite setting is the 2xEL34+2x6L6 via progressive linkage in the power section. This is not possible with the Roadster. Thus, one of the reasons the Roadster is cheaper.
If I sell my RK1, I will miss this power combo/feature...