We don't really know what the future SA interest will be and yet dare you mention UT as an alternative as though the future is more certain than TB and can be defined?
With the unit trust-based shielding method you know with 100% certainty the SA interest rate and can compute exactly how much SA interest you're going to lose. It's ordinarily exactly 1 month of SA interest at the known rate. No more, no less. This part of the equation is certain and known up front.
With the T-bill based shielding method you don't know how much SA interest you're going to lose because the SA interest rate isn't known many months ahead. The T-bill has a minimum tenor of 6 months, so the latter months of SA interest loss are not known. This can be fairly described as "SA rate risk."
With the unit trust-based shielding method you don't know how much you're going to gain or lose while you're holding the unit trust. But the holding period is quite brief (mere days), and you can/should choose a low volatility unit trust to limit this risk.
With the T-bill-based shielding method you don't know whether you're going to get the T-bill or not at a particular bid, but you can specify a minimum T-bill rate that you're willing to accept.
With the unit trust-based method you have much more granular control over the SA withdrawal date and the SA withdrawal amount. With the T-bill-based shielding method you can only choose $1,000 (face value) increments, and the SA withdrawal date can only be approximately every 2 weeks (the T-bill auction schedule). If the cut-off yield is higher than you expect then fewer dollars are withdrawn from your SA, so you may leave more "stranded" dollars than you expect. If you have compulsory contributions streaming into SA it'll be particularly difficult with T-bills alone not to leave excess unshielded dollars.
It's reasonable and logical to take all of these differential risk factors (and the cost of bank queuing for a T-bill bid) into account in deciding whether you should use a T-bill, unit trust, or some of both for SA shielding; when to enter; and (for a T-bill) what bid to place. There's no universally correct answer, and different individuals assess risks differently.