The Picopower chips are a good choice because they give you a lot of low-power options, and they're otherwise quite compatible.
Although I love lots of flash memory, any chips over 32K can't be JTAG debugged by a Dragon. I know the official Arduino IDE doesn't support JTAG debugging yet, but AVR Studio does.
For now I'm keeping compatibility with the Dragon but I hope to move beyond 32K someday. I'd either have to buy a JTAGICE mk II, or perhaps some new third party products that aren't even on the market yet. Most of the third-party JTAG devices don't support the newer chips but I know of two companies working on new JTAG devices that would be cheaper than the JTAGICE mk II, but still compatible with it.
If enough of us request JTAG style in-circuit debugging (single-step, breakpoints, watches) in the Arduino IDE maybe that can be added someday.
Eric