A Corporal isn't really an officer, nor is it a Navy rank. The lowest ranking officer in the modern US Navy is an Ensign. The Navy rank that's equivalent to a Corporal is a Petty Officer Third Class. Despite the "Officer" in the title that's not an officer.
No question. Admiral is better PROVIDED you have the $50,000 minimum required. And, if you do invest $50,000 then the conversion to Admiral is automatic. There really isn't a choice. Above $50,000 = Admiral--below $50,000 = Investor.
It's sometimes hard to see in the daily ups and downs, but over time the Admiral shares outperform the Investor shares by a small percentage. Both are excellent investment choices, but the Admiral shares have lower costs and that is the only real difference.
Take a look at the performance numbers in the prospectus.