I tried both setups, series and parallel plumbing of saddle tanks in my Yellow Aircraft F-15. Series sucked, as I'm very sensitive to out of trim airplanes! Here's why...
At takeoff both tanks are obviously full. As you taxi out the lateral (roll) imbalance begins. By the time you takeoff the airplane is out of trim. This is because both sides were empty upon landing and the airplane was trimmed for that balanced lateral configuration. So after takeoff you might need a click of trim. As it burns you need more trim until empty. Now as the opposite saddle burns down you takeout all the trim you just put in. It's a constant work load and nuisance to say the least.
Additionally, flying around with ailerons or tailerons deflected to accommodate the lateral imbalance causes increased load on servos, linkages, hinges and wings. Aerodynamic handling near or at stall speed is dangerous as the airplane will drop the wing that is holding down aileron.
However, airplanes with a long wingspan are not affected as much due to the increased mechanical advantage.
I always reccomend plumbing in parallel and burning that fuel first.