I ended up buying the MX-H1-AV handpiece as it was going cheap on ebay (got it for just over $100 US), used it a bit at work and definitely prefer it to the RM3E.
Also bought a thermaltronics SHH-1 workstand to go with it after hemming and hawing at the one amazon review that criticized it for not being metal. It was only $60 so I'm not bothered. The only thing I can see as a negative beyond that is that it doesn't have the adjustable rest angle.
I greatly prefer the MX-H1-AV because it is lighter, thinner and shorter than the MX-RM3E. If you like to hold the handpiece as close to the tip as possible (which people generally do), the RM3E has a lot of extra length that seems pointless, even with my fairly large hands. The strain relief is also much bigger and stiffer which means that the cord will pull on the handpiece a lot more.
That being said, the cheap bundles come with the RM3E and the MX-H1-AV only comes with the expensive bundles. I'd absolutely spend the extra for the MX-H1-AV if you're buying the handpiece on its own but I wouldn't buy one if I already had the RM3E necessarily.
Also the MX-H1-AV does go into the regular metcal WS1 workstand.