My first choice is to go on using the Mio for now, there seems to be much development going on in the world of hand held GPS and other may be giving Garmin a run for their money.
I think you're absolutely right. With Garmin going down the Android route we could be in for big changes and new competition. You also have the best maps available (apart from the new MM HD maps), so why not use them if you are happy with the device.
Your SiRFStar III Mio is most likely more accurate than the current breed of Garmins too.