I don't think choosing your website platform based on which Sixbit will implement first is the way to go about picking your platform.
You should choose one based on whats best your your business.
We went with BigCommerce based on recommendations from people with knowledge in these matters. BigCommerce came in high regards.
Cost was another factor and at $250 a month for the Pro plan, it's a good deal.
Building product pages was a learning curve, but now that I've been thru the ringer, I must say the User interface is very good, and managing products is very easy.
The optional integrated payment and shipping gateways implemented with our existing services (Paypal and Shipworks) super easy.
Now that it's up and ruining, I hardly even need to log into the site.. (although I do need to add more products)