If you have time, try both!
No need to go outside for QF F lounge, it has uninterrupted views of the northeast part of the airfield (freight terminal, 16R, T3/T2). Don't know what the platform is to which you refer, unless I've totally missed it in all my visits as far as I know there's no ability to go outside while in the Flounge.SYD QFi J lounge while having windows, and letting light in, does not give you any views to the outside. Views are blocked by the airconditioning ducts. SYD QFi F lounge has a platform that you can go to have views of the outside, above the ducts.
While I have not been into the SYD QF F lounge, (will sometime soon), if you have no access to any other lounges, the SYD QFi J lounge is the only one to get "some" peace and quiet, but if you get other/have other options, use the other options.
For me, views to the outside are a large plus.
I would also like to know about this QF F platform!
Probably, the public access plane spotters platform that can be seen from within the warmth and comfort of the F Lounge. Although, I suspect you knew that already.
Where is that? Underneath the FLounge?
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According to a quick Google, the obersvation deck has been closed for some time....