I think your assumptions are absolutely correct, in that it's almost always easier to just use a cPanel VPS.
Better? Depends on what you mean by that. Your uptime is generally going to be good (though they have had their share of outages), but so is a VPS with a good provider. You're also going to get extreme scalability. You can go from small to a very large instance easily. With a VPS, you're going to be limited by the host node, though if your site is that big, it's almost always cheaper to get your own dedicated server. You aren't going to use a cloud instance.
Cheaper? No way! EC2/Amazon bandwidth is RIDICULOUSLY expensive, and nobody really knows why. Bandwidth costs in most datacenters have been going down over the years, but their prices continue to go up. 100GB, for example will run $9 just for the bandwidth costs alone. The cost for me to offer that to a client in a premium datacenter with a premium bandwidth carrier mix, is less than $0.75.