With Android, given that the Samsung Galaxy S II X is coming out with a very large battery, I don't think you'd want to go with something that isn't as good. The LG Optimus T would be an exception to that and is within your price range. Otherwise the Windows Phone Dell Venue Pro is quite good at $300, although ordering it from the states isn't that easy (for that matter, neither is the Optimus T).
Wind's LG Optimus 2x is also worth consideration, although its price is a bit on the high end, and while its battery life is OK for an Android, that still doesn't mean it's good. Nexus S also isn't a bad phone, but particularly since you've got a 5230 that's working at the moment, there's really no reason not to wait a couple months, both for new Windows Phone handsets to get released and for a few more Androids to be released with good battery life.
The word 'Pentaband' means '5 Bands', from the Greek word 'pente' meaning '5'. For a phone to be pentaband it has to support 5 bands. If the phone has AWS support, it doesn't automatically mean that it is pentaband.
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