Cal.com offers Free Trial, but Planstack does not.
Planstack is designed for Medium Business and Small Business.
Cal.com is designed for Large Enterprises, Medium Business and Small Business.
Pricing for Planstack Starts at $10/User whereas for Cal.com Starts at $12/User/Month when Billed Yearly.
Some top alternatives to Planstack includes Posted, EasyContent, Cal.com, Calendar, neatCal, CalendarX, Qualified, Workvivo, CalendarWiz, Nous, Exly and IFTTT.
Some top alternatives to Cal.com includes Appointy, Calendly, Cogsworth, Kwikcal, Calendify, Book Like A Boss, EasyCalendar, Calendoo, Dotcal, i-Scheduling for Appointments and Calendar.