I don't believe you can go wrong with either, both exceptional cartridges. That said, the F-9 was an improved version of the F-8 meant to work with a greater majority of arms so a bit less compliant overall. The G707 isn't in the ultralight category of the Legendary Infinity Black Widow so will accommodate more cartridges.
Before the Grace I had a Grado Platinum wood body mounted that worked very well until my cleaning lady dusted it into oblivion, along with my Denon DL301. Replaced that with a Grado Blue with an 8MZ stylus, nice but not in the same league as the Platinum.
Next I tried several Pickering cartridges, an XV15 1200E/D1200, and XV15 625E/D625, both exhibited sibilance with the Grace arm on certain records the others had no issues with. All this led me to obtaining the Grace cartridges, glad I did.
I have several Grace cartridges, currently running a Grace F-8L10' Advanced Luminal Trace 10th anniversary edition cartridge on my Kenwood KD500/Grace G707 setup. The stylus are interchangeable so I will at times use one of my Grace F-8C Custom stylus on the arm, both are stunning.
The Custom stylus has a bit more forward presentation over the more laid back Advanced Luminal Trace, both are outstanding. And the synergy of having a cartridge and arm designed to work together can't be denied.
Also running an F-8C Custom cartridge and stylus on the Well Tempered Lab, simply superb. As great as the cartridge is on the Grace arm the WTL shows more of just what these great cartridges are capable of.
on the Kenwood KD500
on the Well Tempered Lab
the F-8L10' Advanced Luminal Trace stylus
the F-8C Custom Luminal Trace stylus
