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Both firmwares are stable as are the jailbreaks
And I doubt there is any noticeable speed difference.
Of course, the speed is limited by the slower processor speed of your iPhone 4
 
5.1.1 because I used to be on 5.1.1 then updated to 6.1.2 but failed because of corrupt blobs. Now I'm on 5.0.1 and it uses LOTS of RAM and often crashes.
 
Unfortunately no :(

I downgraded to 5.0.1 and for the first time everything is pretty silky smooth.When I go from camera to home menu,it doesn't lag at all,it's pretty much like an ios 4.

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Personally, ios 5.1.1 is better.

I downgraded to 5.0.1 and for the first time everything is pretty silky smooth.When I go from camera to home menu,it doesn't lag at all,it's pretty much like an ios 4.
 
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