In any event, I don't agree that state governments, however defined, necessarily are better judges of "the interests of the state" than the people--unless you beg the question by defining "the interests of the state" as whatever the state governments want.
My point is relatively simple: I think that a system of Checks and Balances is important for any liberal democracy to work, and I think that federalism is an inherent part of this Sseparation of powers. Now, on paper, American federalism is quite strong; I like to point out that every American state has its own criminal and civil codes, meaning that each state can impose different procedures and punishments for one and the same crime. But, in fact, in many areas of policy making American federalism was underminded by conditional federal subsidies; Congress hands out money to the states on certain conditions, meaning that states will give up legislative autonomy to comply with federal regulations, because that's how they can receive money from the federal government.
I don't think that the US Senate is enough of a check to prevent this phenomenon; in fact, Senators are not local politicians (i. e. representatives of their state), but local
and national politicians. Of course they have to watch out for the opinion of their constituents (just like their colleages in the House), but those constituents will form an opinion on the basis of national issues, like, for Conservatives, abortion; they will want their Senators to further their interests, i. e. to restrict abortion, in this case. They will thus want their Senators to restrict the rights of the states if that helps to reach the objective of restricting abortion.
If, however, state governors would appoint US Senators, and would have the power to recall them at any time, they would be much more careful about restricting the powers of state governments, because that would affect themselves and their own power; the Senate would be much more reluctant about establishing federal control over matters previously reserved to the states, because the governors appointing the Senators would also be quite reluctant to hand out power to the federal government.