Let's illustrate with some examples. And let's start with the easiest one: the unit trust-based method. We'll assume you're shielding $200K of SA and that the SA interest rate is steady at 4.08%.
Unit Trust-Based Method
Lost SA Interest (1 Month): $680
Unit Trust Price Variation (for the few days you hold it): unknown, but expected to be low
If you have quite bad luck and the price declines by 0.5% over the few days you're holding it then that's a $1,000 loss.
Total result: -$1,680 to +$320 (assuming a +/- 0.5% price wobble band)
T-Bill Based Method
Lost SA Interest (7 Months): $4,760
T-Bill Profit (6 Months @ 3.77% effective net yield): $3,770
Total result: -$990, guaranteed
My personal view is that if your birthday is not at the beginning or end of the calendar month when it's "too close" to move the funds in/out of the unit trust within the same month (and on either side of your birthday) then I think I'd pick the unit trust-based method. Otherwise, I think I'd pick the T-bill-based method (if the interest rate is reasonable). Assuming only 1 month of SA interest loss with the unit trust-based method your average loss ($680 in this example) is lower than the guaranteed $990 T-bill loss (this example). I think that's a very reasonable bet.
Note that you're allowed to combine methods if you wish. For example, there's no harm in placing a "high" competitive T-bill bid when you're still a few months away from your birthday (and the T-bill would mature after your birthday). Your bid probably won't get filled if the bid is too high, but it doesn't hurt to try. Then fall back to the unit trust-based method if you're not satisfied. Or do some of both.