The B650 stand swivels, the A750 doesn't.
The B650 includes a USB flash drive with the user guide on it. (So if you don't know how to power on the TV, you are SOL.)
The B650 picture in picture will work with the analog or digital tuner. The A750 PIP only worked with the analog tuner.
The B650 doesn't have an S-video input, like the A750. (S-Video inputs are history on the Samsungs)
The side and rear inputs move from the left on the A750 to the right on the B650. (looking from the front)
The B650 has 2 USB ports. So you can use a wireless network adapter in one and a USB flash drive in the other.
The B650 has the USB 2.0 Movie/photo/music, InfoLink, Anynet and network connection.
The 40 inch and larger include the Content Library and DLNA network (to access your PC content).
The B650 has the new "Custom" mode AMP fine tuning adjustments.
There is a new (now 4) picture mode on the B650. (A "Natural" mode between Standard and Movie)
The picture IMO is improved slightly on the B650 vs. the A750.
The contrast is noticeably improved
Diagonal line jaggies are improved.
SD broadcast artifacts seem lessened. (Especially the crap around text.)
The menu colors look better.
Disclaimer: I don't endorse any particular brand. Purchasing electronics is a compromise.
Brand selection may not be a main consideration.