There are probably fewer 838s out there to be had. Lots of 750s will be around for a long time. As RCS indicated earlier, the 838 was more of a higher-end unit when it was made, whereas the 750 was the "top of the bottom" in 1977. The 750 was the best of the low-priced receivers, as evidenced by the sheet metal top in place of actual wood, and rear connection extension platform common to the lower-end units. It sacrificed exterior amenities but kept the chassis build quality of the higher-end units. With fewer inputs and controls, it didn't have all the features and amenities of a larger unit. It shares the more simplistic features of the small units in the common power switch, modest phono amps, and more simplistic design of the smaller units, and of course has the lower-end composite case parts.
However, it is by no means a slouch. It has the protection circuits common to the upper-tier units, large heat sinks for the discrete outputs, a pretty good tuner section which can be tightened up nicely, and a more than sufficient power supply. They didn't skimp on the power amps. If It isn't the best, and if you can get past the tin top, it's at least among the best of the 50W rated units of its day in terms of performance.