..For what it's worth........

I use IAR for my 8051 projects and Imagecraft ICCAVR for my AVR projects. Galaxies apart in terms of cost - the Imagecraft compiler is $199USD whereas the IAR is many $1000. For the most part, I'm happy with the Iamgecraft compiler - its fairly mature now and Richard of Imagecraft is very quick to respond - I hit him with a defect many years ago and it was fixed next day. I also suggested register shuffling when doing bit shifts of multiples of 8bits. He also implemented this. Unfortunately, the output of the Imagecraft compiler doesn't seem to compare with the WinAVR compiler. WinAVR, whilst not quite as friendly, seems to do a better job and the code is smaller. However, my first choice is to use the Imagecraft product as it is nicer to use.

As for writing code for multiple targets, I recently was looking at the Atmel sample code for their AT90USBKEY product. Their code copes with their 8051 and AVR targets. From what I can gather using much the same techniques as Bernd describes. Where there a differences, these are isolated by macros that conditionally expand depending on the detected compiler.

The issue you raise is common for a lot of us - I use a variety of ARM based cpus,AVR and 8051.