[Worc]

I would keep the 612. Neither the 3901 or I-12 are as light as the 612. The I-12 is a little easier to clean than 612. The 612 is much eaiser to clean than the 3901. The 3901 only holds four shells total and can't take an extention. The Franchi's both hold five total and can take extentions. All of them have automatic magazine cutoffs with cartridge releases.
If you have to part with it both models you pointed out are good models. A few others would be the Beretta 390,Pintail/ES100, Weatherby SAS, or the Winchester SX2.

 

[Worc]

Good question T! For some reason they keep listing the wrong shell capacity for more than one model. My 612 is very close to holding six shells. I could cut off a few coils and stretch the spring to make it work if I wanted to. Five is enough for me though.
My guess is it's just a typo on someones part.