LOL, no offense but I could never confuse the two. They were both so different, and for one thing Jean Arthur was a blonde. Although their voices do sound similar, that's definitely true. I think Jean Parker is the best Beth. She's not overly tragic, just a sweet, gentle spirit.
I agree with you that Jean Parker was the best Beth but I also agree with the OP there is an incredible resemblence between Jean Parker and Jean Arthur, most especially in semi-profile. Yes they had different hair color but most actresses dye their hair or wear wigs in a few of their movies so it could be easy to see someone thinking it was Ms. Arthur rather than the considerably less famous Ms. Parker. The big difference personality wise to me is Ms. Parker does not have that quirky, somewhat neurotic personality that Jean Arthur does and is a rather conventional though good screen actress.