Underfloor heating and how R values affect it
You're correct to assume that tile will transfer heat more quickly. However, since it's much more dense than wood, it also requires more heat energy to reach a certain temperature. For that reason, heating time is normalized somewhat. 

When discussing energy efficiency we have to look at where the heat can go other than through the flooring. The R-value of the flooring is irrelevant in a scenario where there's perfect insulation below and alongside the heat source. All heat must transfer through the flooring in that case. Therefore, the insulation that was installed below the heat source is the critical factor. If you have a good layer of foam below the heat source, either flooring is probably fine.