Selling first means your next offer is backed by real proceeds rather than a projection, which matters most in a market where financing conditions on your own offer could make you less competitive against a buyer who is not waiting on a sale.
Buying first can work if you qualify to carry both mortgages temporarily, but it concentrates risk. If the Montgomery Place home takes longer than expected to sell, you are paying two sets of costs, and a bridge loan or line of credit against the existing equity is usually the tool that closes that gap.
The right choice comes down to your own financing room and how replaceable the smaller home is if you wait. Get your hand-reviewed valuation from Joel Dyck.