If it was up to me, I would just get the White Rocks. Leghorns are really flighty. Plus both the RIR and the WR are brown egg layers, thus the resulting offspring will produce brown eggs for sure.
Do you have a market for sex-links in your area? Sex-links are really common around here, but most people just get them from the feed store for about $1.65 a chick. Try to think of how much it is going to cost you to produce the sex-link chicks (housing, food, parent stock cost, etc.) and compare that to how much you would want to ask for each chick. If the cost per chick is more than $2/chick, personally I would stick to producing a purebred, dual-purpose breed instead that you could sell for eggs or meat production.