Found it... the comparison of a X50v with X51v link is here ->
http://tinyurl.com/eyhvk
It indicates that using Spb Benchmark Index v1.6, that the Axim X50v is quicker in CPU and File System Index but slower in ActiveSync than the X51v probably due to the improvements in AS 4.x.
Also, following the thread, someone indicates that "...part of the WM5 OS is copied to RAM from where it's executed, that explains why the end users does not get 64 MB of RAM free...".
I knew I remembered reading it somewhere. Just an interesting item to note for X50v users.
Cheers.