Both will be busy days but I'd recommend Tuesday over Sunday. During summer, the opening hours have been extended and it closes at 9pm. Not too many people know about it and therefore it tends to be less crowded already starting at 6pm (many go to dinner

)
I recommend you buy your tickets ahead of time on
http://www.uffizi.org/ (affiliate partner) as it still would mean avoiding any crowds at the entrance! If you wish, once you're at the entrance, ask for a wheelchair for your mom - they have several for visitors free of charge. They can't be reserved ahead of time but if they are available, they will go get one for her to use during your visit.
Enjoy the Uffizi!
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