Microcontroller/processor vs. external controllers

I'm wondering what are some of your thoughts/considerations when selecting a microcontroller with certain controller capabilities vs. choosing an external controller with a general purpose MCU.

For example, let's say a MCU has an Ethernet MAC, or USB OTG. Other than cost, what are some considerations when chosing this MCU vs. an external Ethernet MAC, or USB controller with a general purpose (one w/o a controller) MCU?

I'm considering a design that requires USB, Ethernet, LCD, and Zigbee. I believe loading an MCU with most of these features will require and lot of code space. I'm thinking offloading most of these features to an external controiller will be better. Then I think, why not most of the features? Then I think, maybe it won't be as much code as I think. Anyway, you see where I'm going with this.

Any thoughts would be nice.

In My view this is the order that I follow while selecting a Micro-controller -
1) Speed of operation I mean Core of CPU , 8051,PIC,AVR ,ARM (core for lower to higher speed operation from left to right)
2) RAM and Program memory available for storing and executing the code
3) The Functional aspect like required peripheral are available or not ( Like you said USB, Ethernet MAC, LCD etc...)
4) Kind of package of IC is available ( My choice of chip package are DIP, SOC, TQFP and the BGA ..... as hardware development become complex for Left to right)
5) Dimension / Size and Shape of design
6) Most important is cost

good luck

Integrated solutions are always best.

* An integrated IC will always cost less than puzzling ICs together
* Needs less board space
* Consumes less power
* Takes a lot less layout and schematic effort
* Takes less datasheet reading
* Has much lower BOM cost
* Little room for error, layout issues, compatibility issues.

In addition, not even sure if it's less coding effort. You still need to interface to your external components. But that's not my cup of tea, so I'll just shut my trap.