Make a shortlist of POS providers and compare them on features, hardware, integrations, support and pricing. Look at the full cost, not just the monthly software fee: payment processing rates, hardware, add-ons and contract terms vary between providers. Ask each vendor exactly what is included so you can compare total cost.
Read independent reviews and user ratings, and ask other business owners in your industry what they use. Track your findings in a spreadsheet so you can compare options side by side.