The only thing you are tied in to with Shopify is their hosting which is extremely reliable with unlimited bandwidth.
You've surprised me there. So I'm not tied into the shopify platform at all? So if I have a shopify store and I don't like it for whatever reason, what would the moving process be? Would shopify give me a copy of all my products, images, web files and copy of the database and I would be able to host it somewhere else or give to a new developer so they can do a data conversion and can move in a nice controlled way? What would happen with my domain? Would it be registered with shopify or someone else? Would they transfer it for me? How would the domain name transfer happen
My month on month contract and commitment to shopify are different to the level that I'm tied into them their platform
If you are not happy with NopCommerce you also have to change solution. Whether a web design company is involved is completely irrelevant.
If I'm your developer and your on NopCommerce and want to move to Magento I can't continue as your developer any longer because I don't have the required experience in Magento so there will be an element of transfer and data conversion required. In that regard the process of moving from Nop to Magento and moving from say shopify to Magento would be similar.
Here's the difference and why my opinion differs to yours.
I would give your new developer a copy of the web files and a copy of the database that the store runs on. If the domain name is registered with me then I would arrange an orderly transfer. Please note: I do not recommend website owners registering domain names with developers, all website owners should always register and maintain their own domain name, always without exception.
I would also liaise with the new developers to ensure that there was an orderly handover, that the urls for pages and products could be mapped to their new urls. I'd do this because I'm a nice guy, a professional developer and because even that even though I was losing a customer they would leave with a sweet taste (not that I have ever lost an e-commerce customer to another developer)
I hope this clears up why I think there is a difference and why I believe using a reputable company that uses supported, mature and stable open source e-commerce software gives you greater control and flexibility.
There are further flexibility advantages when your store grows and needs bespoke development like it needs to connect to your non-standard accounting package, or your shop till system or your newly acquired competitors website or you need to sell rope by the metre but you want the user to put in 1.235m of rope (does shopify do this) or sell fish by the Kg but the user enters a part of a Kg (e.g. 1.25kg) you couid change the unit to grammes but you don't want to sell it as grammes, or you want to sell by width, breadth and depth (and then calculate a price based on volume in m3) and keep stock of the piece you cut and the piece that's left, or you need it to talk to your bespoke access database or you want it to have a forum or a blog or you want to create a sub-domain (like go.mystore.com) or you want other retailers to be able to add products or you want a loyalty system and it has to talk to you existing bespoke loyalty system or ... I don't know, these are just a few things I can think of that I've been asked to do. Oh, bespoke reports, that's common, customer's want all kinds of reports, with charts, no not a line graph I want moving charts, I want a bouncing ball that bounces higher the more I sell and then gets fatter the more customers buy it...
I think you might begin to see where the relevance comes in where the difference is. The flexibility is that good Magento and NopCommerce developers have full access to the code base that runs the website, we can edit and update. I have no doubt that shopify will let you say import products (but pictures usually gets a bit tricky in these situations) and maybe export to a csv file (again, pictures). They probably give very good reports, but the bouncing ball one, please phone em up and ask them. I might do it myself for kicks.
I've gone on a bit, I blame it being Saturday and that I've had too much coffee. I'm normally more composed. I may well have convinced you of my opinion, I may not. If you have other things that re-enforce your view, please share them. When someone disagrees with me its an opportunity for learning.
Finally, would you mind considering the OPs question? If you have a wealth of Shopify experience then the OP would probably value your input.
Thanks, and enjoy the rest of your weekend,
Phil