Been looking into Dual Sim phones myself, for the missus to be, She'll be working in the UK and living across the border in the Rep. of Ireland.
She currently uses an S3 and considered the Chinese phones but came to the conclusion that it would be too much of a downgrade.
Then came across the dual sim variant of the S4 but that is so expensive compared to the normal s4. Although now that Three UK have brought out "Feel Like home" the dual sim wont be a problem as she will be able to roam with data for free. without the expense of running 2 phones.
I am on three Ireland and have a similar "3 Like home" when I'm in the North. Calling the UK is also included in my minutes. So that's my situation.