There are 4 main types of ecommerce platform.
1. opensource ( E.G. opencart, prestashop, magento( requires vps or dedicated hosting ), virtuemart, zencart, oscommerce )
2. licensed and hosted by the store owner ( paid magento, actinic, shopify )
3. Saas ( volusion, shopify )
4. Pass ( platform as a service , e.g b2b selling platforms with API )
It depends how much control you want over your store and how you see your store growing as to what product you choose. You have to think into the future, as a short cut now maybe expensive later. Think of the international dimension, high volume, mobile commerce, and hosting requirements.
If you want to control absolutely everything then go opensource. You get the source, you provide the hosting and you can change it to do anything you wish. Moving to another platform will require a migration of data. These can be linked to in-house account systems. Opensource systems tend to develop with trends quicker.
If you want the support of an organisation and their management of your site then a licensed product is the way to go. Moving to another platform will require a rebuild. Some vendors on gaining a critical mass of customers stop developing their products and rest on the license fees. If the product stagnates then the competition passes you by and you are trapped until you decide to rebuild.
Saas products are pay as you go, they are the same as the license product except they are paid for monthly and include the hosting. Exporting data out for migration to another platform is usually not an option. Moving platform requires a rebuild. The advantage is that its quick setup but the downside are the hidden monthly fees that are usually linked to transaction volumes. So the more you turn the higher your saas fees. There are many saas platforms and many different billing methods.
Paas is Platform as a service : this is for B2B where you are looking to link your ecommerce into other systems that your customers have like SAP, Ariba, Punchout2Go. It allows B2B clients to link their purchasing systems directly into your B2B ecommerce platform. This is vital if you are selling into large corporates who have complex procurement systems that they process their purchase orders through. With a Pass ecommerce you are looking for flexible apis that can interface with other systems.
Our advice for control, flexibility and future proofing is an opensource solution. Always think of the next step can I scale to 20,000 sales per month with this system, without blowing up and or costing me a fortune in sales comissions.