A big driver of the right tool is your overall send volume and customer record count. Both of these will also influence pricing.
Even for smaller shops, would recommend checking out some of the big players (eg Adobe, Salesforce Marketing Cloud, Eloqua, Twilio, etc) as they have entry level lower tiers that may end up costing less over time than some of these startup focused solutions (which all seem to nickel and dime you and hit you up with various types of overage charges), and will get you much higher deliverability, automation, and integration capabilities.