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From what I remember reading (and sadly I don't remember the nitty gritty details on this one), GPT as a "spec" is much more precisely defined than MBR ever was. MBR had a lot more variations in its implementation by various vendors in terms of partition IDs and such, so it was more subject to interoperability issues. And I believe GPT gives it somewhat more resilience to issues, such as having a backup partition table in case the primary location is unreadable, which I don't think MBR has. Again, I'm a bit hazy here and don't remember any precise details.
Apart from that, MBR only allows 4 primary partitions. If you want more than 4 partitions, you have to resort to an "extended partition".
And GPT is required for disks larger than 2TB (corrected from original 4TB claim, my mistake).
If you were initializing a new disk, I'd suggest GPT, which Windows now uses as the default these days, because it is a better layout scheme. (The only exception would be if you were using a flash drive that had to be bootable in Legacy BIOS mode and/or needed the storage device to be readable by Windows XP, in which case you need MBR.) But for a disk that you've already got set up....on the one hand, the value proposition is minimal unless you need more partitions or more than 4TB. On the other hand, it's really easy to convert a non-OS disk to GPT....