[U-Boot] weak functions versus conditional compile

I have a quick question I would like the groups opinion on:

What is the rule-of-thumb for defining a function as weak versus using conditional compilation (or #defines)?

For example, I am doing some work on cleaning up the structure and design of the i386 / sc520 code. Specifically, some functionality such as interrupts, timers and CPU reset have more 'powerful' implementations in the sc520.

Should I declare these functions as weak in the core i386 code and use a config #define to override or should I seperate the functions out into seperate source files and use conditional compilation?

Regards,

Graeme