Say there is a close and important question to be decided upon as to whether individual states can make their own laws regarding x as opposed to a federal law saying you must do x (that will theoretically apply to all states). Arguments exist for both sides of the federal vs state issue, so a judge won't look stupid or obviously wrong, logically speaking, regardless of which way he or she rules. Now let's say that his or her personal point of view is that x is a good thing. Meaning he or she thinks it would be a good thing if every state allowed it. Which could only be guaranteed if it was federal law. However, this judge simultaneously felt that the arguments for it to be a federal law were not as strong as the arguments for it to be left to the states. Or there might be the opposite situation where the judge thinks x is a bad thing but thinks the arguments for there to be a federal law to allow it, win out over states deciding.

I suspect that a judge with these conflicting views would be a rare bird on close decisions because most of you humans tend to not look at things completely objectively and will subconsciously find ways to justify a stance that your side has the better argument when, the subject matter provokes strong emotions, when in fact you would think the other way if similar arguments were the about a non emotional issue for you.

But I am sure there are some judges that, even on close decisions where either ruling can be justified, will actually sometimes think that the slightly more correct ruling is the one that goes against their personal preferences.

So, my question is what percentage of these rare bird judges in the US would rule in the way they really think is correct when they can easily get away with ruling the other way, the way that they would personally strongly prefer to win out. Is there a difference according to whether he or she leans liberal or conservative? It seems like people on both sides of the aisle think that judges on their side will rarely fudge but judges on the other side almost always will. What do you think? Is your answer objective?

Your question is imprecise. Judges are not asked to decide whether something is best left to federal regulation or state regulation. Within the confines of the Commerce Clause (which, for practical purposes, is no limit at all), Congress has broad authority to regulate. Very few powers are reserved specifically to the states. Generally speaking, (i) state law can expand on federal law, but it cannot conflict with federal law; and (ii) state laws that conflict with federal law are preempted by federal law. Preemption can be explicit or implicit. (I am putting aside the related question of field preemption for now.)

The U.S. constitution also guarantees various rights. States laws that infringe on those rights are unconstitutional.

If your question is whether the personal views of judges inform their decisions on close questions of preemption or other constitutional law questions, the answer is yes, to varying degrees depending on the judge.