Awesome, thanks for all the info! I'm not a noob to Android but I'm still getting my head around all the Samsung specific stuff
Looks like the best thing for me to do if I get an S4 is flash the pre-rooted ROM, best of both worlds.
No worries mate, it's a weird and wonderful world to explore.
Being a low level Android developer and modding demon, there are 2 things I'd stress VERY strongly.
1. Learn how to use CWM. This stands for Clockwork Mod, which is the recovery system built into MOST custom ROMs. That XDA thread I linked you earlier will have links to download it if your ROM doesn't come with it. You flash it through Odin just like you would a ROM.
The best thing about CWM is it's backup system. It will literally create a freeze state of your device. This is SO invaluable before you flash a new ROM or a mod. Even if the mod you're about to flash is 100% reported as being stable, I can't stress to BACKUP BACKUP BACKUP. If something goes wrong or even if you just don't like how the mod works, it's SO good to be able to just restore a backup and have your phone be 10 minutes back to how it was before you flashed.
Also, if you flash something you shouldn't and you brick your phone (bootloops, crashing errors), backups will go back to before it was broke. Saved me so many times.
2. When rooting/flashing, there isn't a WHOLE lot that can go wrong, especially because Android has become a whole lot more robust recently in being able to recover from crashes and things like that.
HOWEVER, the #1 cause of bricking phones is flashing software that isn't meant for your device. Make sure you check what you're flashing BEFORE you flash it. If your device is the I-9505 (Snapdragon S4), make sure whatever you're flashing is for the 9505. Also goes without saying, but don't flash anything for a whole other phone altogether - I once had a friend who flashed a stock ROM for the HTC One because he liked how the One's interface looked and wanted it on his Xperia Z....
If you have any other questions, I'd be happy to help out. Feel free to PM me if you like.