They're completely different.
Prime still remains on the free hosting servers, doubles the system resources (CPU & memory) and removes inactivity monthly sign-in requirement (as you're paying, of course). It's basically a simple extra.
The memory on your account would end up being 2048MB (2GB) and as for CPU, I don't know so just imagine that as 200% in cPanel.
Premium however uses different servers, high speed web hosting severs (of course that is since there's like over hundreds of people on free hosting whilst on the paid servers there are way less, and some more things), better support for your account, Unlimited domain names, email accounts, databases, disk quota, FTP accounts, and bandwidth (best part of it), SSL usable and domain registraitions, couple of full daily backups, search engine optimisation, no inacitvity sign-in requirement, dedicated IPs are available and much more. The website you would want to look at is
http://x10premium.com/.
Yes, I had to copy & paste one or two as the list is just crazy long.
Note that premium will cost more than prime, but it's worth it.