Not knowing a lot about the Acadamy range of cars I will do my best to try and give you some sort of answer.
As it sounds like you are racing on a fairly slick surface I would be more tempted to go for the centre oneway as this will increase your off power steering, which should help with some rotation on the slower bends as all the braking is on the rear wheels only.
The slipper can be a helpfull tuning aid and will also reduce the wear on your transmission as it helps absorb the shocks on jumps and bumpy tracks. As your track seems to be low grip the wear on the transmission is likely to be fairly low anyway.
Both are worthy upgrades, personally unless you are going through sets of gears on a regular basis I would go for the one way shaft as I would think this will make you faster round the track.
Hope that helps.
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