Though slim, the SG2's 4.5" display support makes the device "near-awkward" unless you have big hands (or long fingers). A thicker device would add to the handling issue. Folks will see what I mean when you hold the device and try operating with one hand.
I would prefer a 4.3" device that is thicker (more battery), rather than a super-slim 4.5" device, since same total size.
Isn't SGSII 4.3" (4.27" to be exact)? Looks like you are talking about Infuse 4G. Rumors indicate Bionic will be 4.5" large and pretty thick device. I hope it's not 4.5" qHD as that and LTE won't bode well for battery life.
Also I don't get the slim form factor argument on SGSII about LTE. Adding just a few millimeters to fit LTE into it isn't big deal for most folks. Still it's fairly slim, very likely thinner than Charge. And who knows if Sammy already engineered smaller LTE chip into it? LTE phone will get slimmer, more efficient eventually.
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