Yoda is pretty smart. I think he already had considered the option, but knew that it would likely result in his demise, and Obi-Wan's as well. Given his dialogue about the future of the twins, then one must assume that he also knew that they would need training, protection, and guidance. If Obi-Wan and himself were dead, then who would give Luke all that wise Jedi talk?
Thus, the risk would be too great. Being a prudent, wise person, Yoda chose the cost/benefit option most suitable to his cause.