More like 2 dollard dirtwalker. Do you also have a 375? What make is your O/U 450/400?well, since you are asking for opinions, here's one.
first, having read some responses, i think red leg has a viable argument for a 375 flanged (other than acquiring brass) it is completely capable of killing DG.
I have an o/u 450-400. i love the gun (very accurate for a double). i have shot blue wildebeest and moose at 200 yards and beyond (my double is scoped) and the 450-400 is more than capable for such shots. so trajectory concerns are not really worth considering unless you are shooting much further.
there is no question that the 375 is lighter than the 450-400 given the figures you showed, so.... are you going to do more PG hunting or DG hunting with said rifle? both guns shoot to a fairly similar ft/lbs of energy so the 375 should actually recoil more. the question is, do you want more thump from the 400 gr bullet or a flatter shooting rifle (slightly) with a 300 gr bullet?
I guarantee that up to 200 yards (with both scoped) there are not enough differences to matter for trajectory. do you want to carry the extra 2 pounds? do you want more available ammo and brass? the problem with the 375 is... you lose some of that calibers versatility with a double, since it is unlikely that your double will shoot 250 gr and 300 gr bullets to the same point of impact. i used solids, softs and percussion point (north fork softs made for cats or light skinned animals) and all 400 gr. shot to same POI. my rifle regulated to 400 gr bullets at 2100 fps (hornady bullets regulate to the same POI in my gun)
so, all this said, if I was picking one, my contact with DG would have some influence, but, i feel completely confident of my rifle out to 250 yards. my 400 gr bullet is only going about 1650 fps at 200 yards, but, it shot thru a moose and a blue wildebeest at that range!
ultimately it is your decision. a more trim rifle, or a slightly heavier with a touch more punch. either way, you are going to buy a fine rifle. i suggest you get it set up for a 1-6 scope no matter which rifle you buy.
there it is, my 2 cents!