First of all, you should never order an RMA just because there is a "new and improved" model out if you wouldn't have ordered one otherwise. If the rattling of the buttons are truly annoying, get an RMA. But if it suddenly started bothering you after you hard/read about this "new and improved" version, then keep your phone.
Second, there are no guarantees that you are going to get a new device with any of the hardware "updates." You may not get bigger speaker holes, or a more flush SIM tray, or whatever it is you are looking for.
Third, there are other variations on the devices. Some devices may have a yellowish screen from what I hear. Bottom line: if you think your RMA device will be identical to your current device except only for the changes/updates you want, you may (or may not) be in for a surprise.