I have tried both of these. Both are great ROM's. I am currently on Mammon's for personal preference. I have had no stability issues what so ever.
-I am still on RFS, so I cannot comment on the ext4 stuff.
-I have the stock SD card.
-No programs on SD card unless it does it automatically.
-Wifi tether works amazing, I use barnacle free though. the android-wifi-tether was too flaky for me. barnacle allows me to work off the wifi connection for a long time ( I have tested up to 5 hrs without problem, haven't had time to test longer)
I don't have performance programs to test. From real world perspective, Mammon's seem slightly more stable to me. And what I mean by that is, it feels snappy all the time. Sometimes with Viper, when I would go to make a phone call, I would get a tiny bit of lag etc. I don't know if it was me or the actual rom. I wouldn't take anything away from Viper though, it is a really nice rom.
All I know is, with either Viper or Mammon, my phone was running far better than it ever was at stock or any 2.1 rom, and it had more features! These guys make high quality, feature rich roms. They should be working for google or samsung or something.