If you will end up in PAL you should try: 720p25/60. The camera it self will make an amazing 25/60 (41,666%) slow down. Then if you need more slow, as you say you can do it with Shake, Motion or in FC with Twixtor or any other good speed plugin.
As Noah says, you could get even a lower speed slowmo by recording 720p24/60, but this would complicate your workflow going to PAL.
Mac OX 10.5.2-FC 6.02-QT 7.4.1
G5 2x2Gh 4GbRAM-BlackMagic Extreme
PMBP 17"Core2Duo 4GbRAM-AJA ioHD
SONY EX-1 . SONY PD170
..and always a big mess on top of the table.