where on earth got symbian 40

s40 and s60 is totally different thing, s60 is powered by symbian while s40 is NOT AN OS...
Duh   if dono don simply say....

s40 is using icon based menu and the icon is samewith s60 that doesnt mean its s60....
Again S60 and S40 is different, s60 FP1 = symbian OS 9.2 while s40v5 is newer compare to s40v3

s60 is OS therefore the phone function can be expended using software while s40 cannot...
some difference like voice recording in s60 is only 1minute while in s40 is unlimited. This is because s60 is an OS, nokia want more people to write or purchase software to increase its capability. Therefore it limits certain feature in the phone like voice recording, however, installing software can record unlimited sound in s60
S40v3 and S40v5 difference is only the music player. Not other sinificant difference.

And s40 doesnt support multitasking
Yeah, s60 doest mean symbian, s60 is a range of phone by nokia that use symbian OS. Just because nokia s60 is famous, so every though s60 represent symbian. some even thing s40 is symbian 40

Btw, sisx or sis is just a container or installer.

S40 has all the basic features like unlimited recording, flash sms etc.
s40 indeed stable and fast. But it doesnt support multitasking, gaming cannot listen music.