Well, they blocked the exploit as of 5.1, so someone needs to find a new exploit. It's entirely possible that there's an exploit that exists in 5.1 but not 6.x, but it's also entirely possible that there's an exploit that exists in 6.x but not 5.1. We won't know until someone actually finds one.
But, unless you have dumped your NAND beforehand, you can't downgrade to lower firmwares, so if you're trying to decide between staying on 5.1 or going to 6.x, stay with 5.1. You can always update later if need be.
This is actually incorrect. The exploit that Gateway uses is present all the way to 6.2, they just have to edit the necessary code to make it work.
Source Email from Gateway Team:
"it is just requiring some work, but it will be done, as the same exploit is there on 5.x and up to 6.2"
Before anybody says it, I'm not helping Nintendo patch it by saying this now that Gateway is in the wild.