I'm going to sound stupid here, but I've never understood exactly what you guys mean by a custom ipsw. I've assumed that you guys modified the ipsw somehow, but how you did it I'm not sure. However, that may be what PwnageTool does, and I've never used it. Anyway, I have a Mac, so explain what I'd do. BTW, I used Redsn0w to jailbreak, as suggested by your guide.
Speaking of your guide, why do you suggest PwnageTool, when your guide had Redsn0w?
Glad you think of me as a friend. Same here.
Sorry for the delayed response; I've been offline.
PwnageTool is a program which modifies iPhone firmware files by applying all of the modifications usually done by a jailbreaking app to the ipsw file. The resulting file can be installed on any phone which has already been pwned, resulting in a one-step jailbreak (PwnageTool can also pwn a phone which has never been jailbroken, providing it has the old bootrom).
The primary advantage of PwnageTool is that it is able to remove baseband upgrades from firmware updates so that one can upgrade without losing the ability to unlock. For instance, you could use PwnageTool to create a custom 3.1.3 firmware without the 5.12.01 baseband upgrade which would make your phone incapable of unlock via blacksn0w.
Another benefit is that this method can patch changes made to the firmware by Apple which close exploits. This means that once your phone has been jailbroken, it can always be jailbroken. There is nothing that Apple can do to firmware 4.0, for instance, that would make it impossible to jailbreak using PwnageTool
even if phones already updated to 4.0 or that came with 3.1.3 could not be jailbroken.
For owners of 3GS iPhones, custom firmwares also have the advantage of being installable even if Apple is no longer signing that version of the firmware. This is because the initial jailbreak patches the signature check out of iBoot. A 3GS owner can no longer restore to the official 3.1.2 unless he has his SHSH on file, for instance; if he is jailbroken, however, he can always restore to a custom 3.1.2 firmware.
Since your phone is a 3G and you probably have already upgraded your baseband to 5.11.07, and since you do not want to upgrade to 3.1.3, the primary benefit of PwnageTool for you is that it is a one-step solution. Once you have created your custom firmware, you will be able to restore to it any number of times without needing to re-jailbreak your phone.
It all comes down to personal choice, especially for a 3G owner. There is nothing wrong with using Redsn0w, and it is certainly easier for a beginner.
If you decide to use PwnageTool, as an AT&T subscriber you should be sure to use Expert Mode and *uncheck* the box for activation.
