got 1, usually Sense ROM bigger. ASOP rom smaller after installation. you can check the /syatem size to compare each time you try different rom.
rooting, partition, flash any rom, flash recovery, etc.. anything wrong, no worries. as long you have pc and usb cable
but S-OFF is more advanced thing, is actually modified the HBOOT, something like board bios. if the moment S-OFF, ur pc hang or power off.
possible it gonna brick the phone. but if you follow the steps carefully, with latest version of s-off which are very stable. it's not scary at all.
so, understand first the advantage of s-off, are you really need it and understand? then go ahead. for me, I s-off so we can choose the recommended system partition or custom table we like. I have 303MB internal memory, happy with it.. compare to stock user, only have 147MB internal memory.
Say, now we jump to AOSP or Gingerbread ROM, which are generally smaller than Sense ROM, we can take advantages of their smaller size and get S-OFF to resize the internal memory. Then, when we want to get back to Sense ROM, we will have to adjust again? (I am thinking that it's a yes, but just need some confirmation.)