Is you 8051 serial channel consumed? Possibly the dataset (rs232 lingo) responsibility means you can't risk muxing between two comms and possibly missing something...

Maybe you could use the 8051 serial interface and pipe it through a simple MAX232 varient and handle your on board serial comm (assuming it exists) through a bit-banger port? Or, if you really want to have some fun, use a dual uP design and use a low pin atmel with a PIC and share the load. That's got some interesting advantages to compensate its added development burden.

Of your listed choices, I'd favor the MAX311xE as I'd consider it an investment in future designs. I'd prefer the part have a multi-byte buffer but as you said, you've got all the uP time available for comm support.

-ping