Transmission Service/Flush

[268i]

Hi All,

I have an 06' Yaris with approx 45,000mi on it.
Although this seems quite low, 99% of driving is in the city, stopping and starting. Hence at the most recent service (not at the stealership fyi) I was told that i'd be good to have Transmission Flushed/Serviced.
Quote for $280 (New Zealand Dollars) using Genuine T-IV Fluid.

(I seem to see a lot of others, in the US, using WS fluid. However, on the dipstick, T-IV is what it requires.)

I email a local "Oil Changers" chain for price quote and I get this email response:





The latter being considerably cheaper, i'm inclined to go with it.
Should I stick with using Genuine Toyota ATF? Any benefits from using a Snythetic WS, or WS Spec'd Fluids?