7 Major Factors which helps to choose the Right ecommerce Platform are as under

1) What and How / How much are you selling.

Product Type : Digital or Physical Product

Payment Types based on Business need.

1. Onetime payment
2. Recurring Payment
3. Authorize Now & Capture Latter (Free trail + Charge latter )
4. Offline (COD, Cheque Money Order, NEFT etc.)

Volume High Volume, Medium or Low
% Transaction fees of the Platform + Merchant Account % fees

2) Can you make it look Good yourself or need help?

1. Readymade Templates / Themes
2. Custom Designed Theme
3. Customization in Functionality

3) Standard Features Availability?

1. Mobile Friendliness
2. Level of Customization Possibility
3. Ready made Payment Gateway Integrations
4. Easy to Manage the Backend.
5. Integrations with Accounting, email Marketing tools, Shipping Label, Tracking, etc.

4) Is it Secure?

1. PCI Compliance
2. Customer / Order Data
3. SSL

5) Is it Scalable (Grow with you ? ) ?

1. Manage Peak Traffic levels

6) Is it Affordable?

1. Monthly v/s One time

7) Will you get support you need ? Documentation / Community Support 

1. Hosted one should have Good Support and Documentation
2. Open Source to choose Magento, Prestahop, woocommerce, Opencart, Xcart, Zencart or OsCommerce