The gauge is an opening question
Knitting for Olive Heavy Merino publishes 18 stitches and 30 rows per 10 cm. Cascade 220 publishes 18–20 stitches over 4 inches (10 cm), with no row gauge captured. Both include an 18-stitch point, but those source labels use approximate paired units. Make and measure a sample to the actual units in your pattern.
Merino and Highland wool are not the same specification
Heavy Merino identifies 100% merino wool; Cascade identifies 100% Peruvian Highland wool. We cannot establish equal softness, drape or stitch definition from those names. Both call for cool or cold hand washing and flat drying. Construction is unverified in this dataset, which is an additional reason to compare the actual washed samples.
The bigger hank contains less length per gram
Heavy Merino offers 125 m per 50 g, equivalent to a calculated 250 m per 100 g. Cascade 220 offers 200 m per 100 g. For an illustrative 1,000 m requirement, that is eight Heavy Merino balls or five Cascade 220 hanks before adding a swatching allowance. These totals say nothing about how much fabric each yarn will produce at your tension.
Before buying a garment quantity
Heavenly Yarns lists Heavy Merino and Yarn.com lists Cascade 220. Check the current shade and dye-lot quantity directly. Pay particular attention to the height of a washed sample: the Heavy Merino row specification is unusually important for a pattern with a fixed row count, and Cascade’s absent row figure cannot be substituted with a guessed value.
Specification sources: Knitting for Olive Heavy Merino. Cascade Yarns Cascade 220. Sources and retail evidence are detailed below. These are desk comparisons; no physical testing has been performed.