@Crash Thanks for reaching out!
Based on the specs on the Suunto Website, the only differences we can see between the MC 2G and the MC 2G 6400 are that the MC 2G 6400 offers a direction scale in mils and has USGS UTM scales, while the MC 2G has direction in degrees and has Metric UTM scales. The MC 2G 6400 is listed as a 'Military Mirror Compass' and has 'Rugged construction' whereas the MC 2G is listed as 'The Sighting Compass for Global Mountaineers' and is 'the advanced navigation compass'. It does not look like the MC 2G 6400 is currently in stock.
In regards to your Silva 15 Ranger compasses: Silva is a different company than Suunto, so you would need to reach out to them regarding their warranty. You can find their contact info on their Silva Warranty page here.
Hope this helps!