I think Obi Wan always hoped to teach Luke the ways of the Force eventually, but that wasn't especially important at the time they were deciding what to do with the twins. The priority at the time Luke and Leia were born was not to rebuild the Jedi Order - the much more important issue was to protect them and prevent the Emperor from discovering that they were alive.
It was also useful, in pursuing the goal of anonymity for the twins, to keep them far apart from one another. That way, if the Emperor found one of them, there would still be a chance to keep the other one hidden.
The secondary concern was to make sure the kids had good homes and loving families. Owen and Beru were decent people, regardless of Owen's feelings about Anakin, the Jedi, etc., so Luke would be safe and cared for with them.
And the Organas were already thinking about adopting a child (I can't remember if he said that they were unable to conceive a child on their own, but it is certainly possible). They were a respected and loving couple, and were obviously very friendly with the Jedi and the cause of restoring the Republic, so there was no reason not to let them raise Leia, and every reason in the world to entrust them with Leia's upbringing.
Leia was relatively safe with the Organas because they were very high-profile people, royalty in fact, and even the Emperor was wary of the negative publicity that would result from any attempt to kill the ruling monarchs of an important planetary system (at least until he had an operational Death Star which made it less important to maintain popular support). In a way, the visibility of the Organas made it possible for the remaining Jedi to keep an eye on Leia from afar.
Owen Lars was a simple man living in a remote area on a desert planet in the middle of nowhere. He was very low profile and not visible at all. That meant that someone would have to stay close by in order to keep an eye on Luke. Obi Wan volunteered for the job, presumably because of a combination of factors- his close relationship with Padme and Anakin, the guilt he felt for the way Anakin went astray under his tutelage, the fact that he (presumably) thought he had killed Anakin, and on a more practical level, because he had very little else to do with the rest of his life.
I never got the impression that Obi Wan didn't care about Leia, or cared more about Luke - it was a pragmatic decision, as I see it. Leia was safe and secure with the Organas. Luke was less safe and secure, because he was on Vader's/Anakin's home planet and living with Vader's/Anakin's half brother.
It was not really about which kid Obi Wan liked more, it was about what would be best for the kids themselves.
Of course, it was also fairly important to make sure that at least some of the Jedi survived. This is why Yoda went to a remote swamp planet in the middle of nowhere. If the babies hadn't survived childbirth, Obi Wan probably would have gone somewhere as remote as Tatooine anyway, just to stay out of sight.
It was fortunate, therefore, that Obi Wan could kill two (or three) birds with one stone, so to speak: He needed a loving family for Luke. He needed to keep an eye on Luke. And he needed to live somewhere out of the way, somewhere remote and far away from "Imperial entanglements". All of these goals would be achieved by bringing Luke to his aunt and uncle on Tatooine, and living somewhere nearby.
And not to put too fine a point on it, but Leia was already taken care of. Since that left only Luke to worry about, Obi Wan stepped up and took responsibility for his safety.
As for your sub-question, I don't think Obi Wan began training Luke solely for the purpose of going to Alderaan. Even if the trip had gone as expected, and Alderaan was still there, and they got there and discovered that Leia had escaped from the Death Star herself, and everything was just fine, Luke had no reason to go back to Tatooine. He was going to become a Jedi no matter what else happened. Since the trip to Alderaan took a few days, and there was little to do on the Millennium Falcon (aside from playing that weird holographic chess game with Chewbacca, which might cost you an arm if you beat him), it made sense to start the training then and there.