The below is some comparison information which should relate to both cards, in general from what I can see the 555M seems to improve gaming perfromance by about 15-20% based on alot of reviews out there so if affordable is probably worth it:
EDIT: I think each card has different versions so will depend on what version PCS stock as to the performance of it and how it rates above. However from what I can see in general the 555M is worth the extra money for the better gaming quality it will bring.
I wouldn't know the answer to that one, may be worth looking around the forums for the reviews of the 2 laptops to compare the comments regarding the chassis and the overal feel of the laptop.
Alternatively current owners may be able to comment on this thread letting you know their experiences.