OK - guilty of assumptions. (Some)
I am relying on info for others as far as the questions of age and future development goes. However, as you and others have mentioned, most of the activity is in the 300 series.
And, as others have verified, the 300 has some (but of course not all) faster and more powerful processors.
Very few applications need redundant CPUs, hot standby, etc. If you do, then obviuosly you use a platform that has it. If you don't why spend extra money and consume more panel space?
The communications quirk I had was on a mixed 300/400 system. When the comm cable was pulled all memory in the block was zeroed out instead of holding last values the way other manufacturers I've used do. This may not be exclusive to the 400, but I hadn't experienced it on 300 only systems.
I'm a little surprised by the antagonistic tone, Ken. I don't think I was bashing either platform. On the other hand, if I'd expressed my full opinion of Siemens you'd have cause to be offended.