Hello Graham,
Both aMSN for Mac OS X and Mercury Messenger for Mac are still being developed. I use neither, but I did test older versions a year or more ago. Based on those old tests, your description is good one for both apps. However, both can offer video support, which MSNM for Mac NEVER does.
For Mac users who must video with a PC that can
only use MSN/Microsoft Messenger, I recommend aMSN or Mercury Messenger, which ever works best for you. Either can connect, so I provided links to both apps. Depending upon how important your "MSN" video sessions are, it may even be worthwhile to install both and use whichever works best at a given time.
If both you and your PC friend are willing to install your respective versions of another software application, based on my recent testing
Skype seems to be the easiest and most reliable, free, cross-platform video chat application. Skype also offers audio with its video. In my tests, Skype works better for cross platform video chat than any of the apps that were developed only for PC (including MSN/Microsoft, Yahoo!, or AOL's Instant Messenger.)
If you are looking for other options, there are also some other cross-platform video chat and video conference applications listed in
Some Applications You Can Use With iSight.
Cheers,
Jim
Mac OS X (10.4.9) G5 DP 1.8 External iSight