Wow, you have ton of products bro. Personally, I have used both Volusion and BigCommerce for my own stores and I favored BigCommerce as its 1) Overall cheaper 2) Has unlimited bandwidth.

I also used both platforms to build stores for my clients and about 75% of them favored BigCommerce mainly because of unlimited bandwidth, navigation, design, and tools. At times, Volusion lacks quality support that is a must for this type of business. If I was you and had that many products, I would def. look at something with unlimited bandwidth as you will probably have a lot of traffic based. I think Volusion only gives you 40 GB of bandwidth with their most expensive plan and charges about $7 per GB once you cross your monthly limit.

Hope this helps and good luck with your expansion!