The 424 is about 6 years old and the 737 is about three years old and has a blueray player which the 424 doesn't seem to have. Can't comment on whether either is good value for money since I can't know what prices you found on used HTS systems.
In general for home theater a separate receiver and speakers will be better than an all in one HTS package which usually can't be upgraded and tends to be less reliable in the long term.