I opened a thread some times ago on MSDN Forum "exclude msprop:design-time-name at comparison" that was Archived.
The solution suggested there was to use BIDS Helper SmartDiff.
Today I found another workaround, after downloading a COPY of both versions of a .dim file, I can open them and replace the regular expression:
with anything (for example an empty string)
This way when you use a compare tool you will only get the "real" differences showing up.
Link to archived thread :