Considering they haven't released Handset 4G pricing yet, I would say that if you live in a 4G area I would wait to see what the pricing ends up being.
Considering the 4G pricing that was released is for, data-only devices and is actually cheaper than current 3G pricing for the exact same device ($59.99 for 5gb of 3G vs $50 for 5gb of 4G) I think there is a good chance that 4G handset pricing will be a minimal increase over current plans ($10 premium would be my guess).
However, if you don't live in a 4G area, or near one in an area that is likely to be an early expansion area, I would just wait til the dual core phones come out and grab whichever one you fancy. If you are on a $49.99 or higher base plan (so the talk and text $59.99 plan or higher), you will have an upgrade in 12 months when 4G will likely be much more widespread.
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