Assumption: 1 AU MR = 0.65 US MR, AUD - USD FX = 0.65.
$1 AUD = 2.25 AU MR = 1.46 US MR (US Amex)
$1 AUD = 2.25 AU MR = 0.625 Aeroplan points (via MB xfer)
If you're spending up to $100K PA, this translates to
US Amex: 146,000 Aeroplan Points
MB: 62,500 Aeroplan Points
Entirely up to you whether it's worth it. You'll need to take into consideration the costs of the extra points etc.
If we take fees into account:
2% (1% for Amex FX and 1% for Wise):
$100,000 AUD on a x3 category (Direct Spend on US Amex) = 65,000 USD = 195,000 US MR/Aeroplan Points.
$100,000 + 2% = $102,000 AUD for 195,000 US MR/Aeroplan Points.
$100,000 AUD on general spend (Direct Spend on US Amex) = 65,000 USD = 65,000 US MR/Aeroplan
$100,000 + 2% = $102,000 AUD for 65,000 US MR.
This also applies to general foreign spend.
$100,000 AUD on general foreign spend (on AU Amex) = 225,000 AU MR = 65,000 Aeroplan Points
$100,000 + 3% = $103,000 for 65,000 Aeroplan Points.
$100,000 AUD on AUD spend (Xfer via US Amex) = 225,000 AU MR = 146,000 US MR/Aeroplan Points
$100,000 AUD on AUD Spend (Via Marriott) = 225,000 AU MR = 65,000 Aeroplan points