I'm trying to discern why I would want to "upgrade" at all.
People who are using iPhones that are a couple (or more) generations old always have to ask this question every time an iOS upgrade comes out that still happens to support their device, because most of the new features are turned off for the old phone models. For example, turn-by-turn navigation was supposed to be THE new compelling feature of Apple Maps...but if you have an iPhone 4, you don't get to use turn-by-turn, OR Flyover. From that perspective, Apple Maps is truly a "downgrade" for iPhone 4 users: you get all of the downsides with none of the supposed new benefits.
However, there is always one somewhat compelling reason to upgrade to every new iOS release that you can:
You can run apps that require iOS 6.
App developers have a tendency to require close to whatever the latest version of iOS happens to be at the time of their app's release. And updates to existing apps will quite often ratchet up the minimum iOS version requirement compared to earlier versions of the same app, so if you don't upgrade iOS, you won't be able to upgrade some of your apps.
Marco Arment's
The Magazine is a perfect example of this: it is, in essence, just a reader app, and is not CPU, GPU, or memory-hungry by any stretch and will run perfectly fine on a 3GS, but it makes use of APIs that are only available in iOS 6, so if you want to subscribe to and read The Magazine, you have to be on iOS 6.
If you are going to upgrade, though, you should think LONG and HARD about that decision. Because unless you took some very specific steps BEFORE iOS 6 was released by Apple, once you upgrade, you will NOT be allowed to go back. Apple actively blocks iOS software downgrades on iOS devices,
which I think is completely awful of them.
(...and which reminds me of another good reason NOT to upgrade:
if you don't use an "Apple-approved" carrier, your life on iOS 6 will suck.)
As for being able to restore, you've got it unlocked from ATT, right? (*if you're on ATT?)
If so, jailbreak it, get your blobs set aside from Tiny Umbrella, you should be able to restore to 5.1.
Whoa, whoa, whoa...you are confused about a great many things here.
First, unlocking (at least in the context of official, sanctioned unlocks) and jailbreaking really have nothing whatsoever to do with each other. You can do one without doing the other, and accomplishing one does not make accomplishing the other any easier.
Second, you can only "get the blobs" for your device for whatever the CURRENT version of iOS is at the time you try to get them. When you "get your blobs", you are not ripping them
from your device, so the version of iOS that your device is running right this second has nothing to do with anything whatsoever. You are downloading your blobs from a server that lives at Apple.
This means that unless you already "got your blobs" BEFORE iOS 6 came out, you can't get iOS 5.0/5.0.1/5.1/5.1.1 blobs for your device, because Apple's servers aren't handing them out to anyone anymore. That ship has sailed, and you weren't on it. Thus, downgrading is not possible.
And, yes: it
is a terrible and draconian system.
-- Nathan