In fact, if you want to work with the original INTEL 80C51 or 80C31, they are the same. You can hook up the EA pin on both so it can use external memory.
80C51 has internal factory programmed ROM, but I have found INTEL 80C31 stamped devices which even had internal ROM (maybe some bulk made types Intel had made and they sold them stamped as 80C31 because in normal use of 80C31 you configure EA for external ROM?)
I found this out with an external eprom from which i boot the controller. After booting i switched the EA pin, so the internal rom was accessible for the program in external eprom. The external program putted out the bits in the internal ROM via the serial port. OUGH... why did I do this? Just for fun. The internal ROM wasn't protected.
I used a disassembler and peeked at the disassembled code (only 4K.... ;) .
I want to say something more:
A 80C31 doesn't neet 'a lot of' external components, only 2 in fact (an address latch and an eprom). If you say it needs 'a lot of extra board space' and a lot of wiring, I agree :)
At times when 8051 derivatives with internal flash rom where not easy to find for amateurs (I won't speak about the VERY expensive parts with UV-erase windows :( ) I used myself a few microcontrollerboards with 80C31, SMD-latch and eprom. I can say, I didn't have less fun than I have now. Of course, it IS easier now. And you HAVE more I/O pins if you use internal memory. But in the (few) cases you need much SRAM, you still have to use the latch and an external SRAM chip.
Look at some devices like modems. There are still many types out with an 80C31 derrivative (like Winbond or so, at higher clockspeed)and EXTERNAL flash EEPROM.

Regards,

Henk