The receiver is totally different. The 732 is based around the idea that you have 1 probe measuring pit temp, and 1 probe measuring meat temp. So the pit probe can alarm outside a range (like beep if the pit is below 200 or over 250), the meat probe only alarms above a temperature (like beep above 195).
The 733 has more flexibility, either probe can be a pit or meat temp, and it even ships with the newer hybrid probes (shorter than a meat probe, but with a pointed end). It has a few convenience features, like you can adjust temperatures up or down instead of only going up until it wraps around. Downside is the temperature numbers are harder to read, the LCD on the 732 is much clearer with bigger numbers. You can see it more easily across the room.
It's really up to you which you want to buy. They're just different, one isn't necessarily better than the other.