given the higher crotch strap on the signo as well as those massive shoulder belt pads, i'm doubting that the signo can work for a small baby. firstly, the buckle will be near the baby's chest. secondly, those shoulder pads may be too big to allow the chest clip to be properly positioned (i'd have to double check the manual to see if the seat can be used without the pads, mostly likely yes). thirdly, with the longer crotch strap and harness set to lowest height, i'm not even sure the harness will tighten all the way. i.e., the splitter plate might end at the adjuster mechanism before the harness is snug. this is just speculation though. overall, i just don't think it will work for a newborn or young (<10-12 lbs) infant.
as for the twisty straps. they are slightly narrower and therefore slightly twistier than those of a MA, but they don't twist through the buckle tongues like they do on the como.
i would definitely give the signo to your 3 yo and the MA to baby. our slightly chunky 3yo actually fit very well in the signo, and i actually
really like the way the long buckle fits around her waist. definitely better for older kids than infants.
#1 hi res pic of 3yo in signo
#2 hi res pic of 3yo in signo