judging by pics alone, the tanaka looks to have better plastics than the maruzen (compare plastic butt stocks and pump grips from various retailers). I was surprised that an unloaded tanaka m870 weighs more than an ics mp5 a4 with a large 8.4v batt. and it is a very noticable weight difference.
Also, it looks to me that slugshot shells may be able to be fired from a standard m870 with some minor mods (remove inner barrel assembly, mod that ring thingy at the front of the mag tube...) although that is not confirmed.
I would go for the tanaka as as it is that bit more powerful and more reliable on propane, plus it has character. what other airsoft gun has accesories with names like
super magnum