I replaced my MT3G with the Optimus and so far the improvements are worth it. It's snappier, seems to have better signal and call quality, and it's form factor is comparable to the MT3G. Overall, it's a nicer device.
I also replaced my MT3G with the Optimus T, and even though it has a slightly faster CPU, the additional RAM makes all the difference. Although, I miss the notification light and the physical call buttons, I welcome the power button on the top.