When it comes to resizing, the pricing is the same as you have the option. You'd need to pick the one that meets your specific needs. The two options may appear the same, though they are different.
When resizing CPU and RAM Only, you can scale up or down. You can start with 512MB and scale up to 1GB, then back down to 512MB. You can scale from 512MB to 8GB and then down to 2GB. There's more freedom when it comes to this specific option. You do sacrifice disk space though.
When resizing Disk, CPU, and RAM, you do not have the ability to scale down, only up. This is due to the disk resize that takes place. Resizing the disk up is safe, though resizing down, not so much. With this option, if you scale from 512MB to 1GB, your stuck with 1GB and can not scale back down to the 512MB (because of the disk resize). If you scale from 512MB to 8GB, you're stuck at 8GB (again, due to the resizing of the disk). This one offers a little less flexibility, but provides you with the increased disk space.
You can, however, mix the two.
If you start with a 512MB and you're absolutely sure you need to full deal, you can upgrade Disk, CPU and RAM to 1GB. From there, if you don't need disk but you do need resources, you can scale only the CPU and RAM to 2GB, 4GB, etc and then back down to 1GB.
There's a lot of options, though it all boils down to what you specifically need. Some applications don't need the Disk as much as they do the CPU and RAM, so there's an option for that. Some need a mix of both, and others only need the full deal.