What type of Rooster should we get?

[ASHFarmChickens]

We have a mixed flock of Rhode Island Reds and White Leghorns. What type of rooster would breed with both breeds and produce more egg laying hens?

 

[ChickenCanoe]

Either.
Those are 2 of the best egg production breeds. I guess it depends on whether you want offspring to be pure leghorn and L/RIR cross or pure RIR and L/RIR cross/. The latter will be larger and a little less efficient in feed:egg conversion.
And do you want more brown eggs or more white eggs.

ETA
I'm assuming your question was which of the 2 breeds. You could always add a different breed as well. Perhaps Australorp or Dominique, both excellent laying breeds.
Or other excellent production breeds that would also be great flock protectors are Penedesenca, Minorca, Hamburg or Fayoumi.

 

[donrae]

Do you have a preference for egg color? Is feed cost a big concern? Do you eat your excess cockerels?

Those are the questions I'd ask myself in your situation. Egg color's pretty self explanatory. If feed cost is a big issue, go with the Leghorns. Nothing churns out eggs on small amount of feed like those girls. If you eat your cockerels, go with the Red. Leghorns don't have hardly any meat and butchering them seems laughable at times.