What is the difference between these two Primes? I had always thought they were the same, but more of what I see on the forums suggests this is not the case. If they are different, I have no idea which one I have but would like to know. Thanks.
I think you mean 20th anniversary. The 25th anniversary OP was a G1 reissue. The only differences between the Hasbro 20th anniversary version and the TT MP are: No trailer, battle damage paint and shortened smokestacks.
Hasbro has released MP Prime at least twice that I can think of. The first release had all the die-cast metal of the Takara version, the second release was all plastic. I think the second release is the one often called DVD Prime, since it came with a DVD.
They never put out an all plastic MP OP, but there was one with a DVD. The only difference between the releases is the DVD version lacks the battle damage paint. Optimus Prime (G1)/toys - Transformers Wiki
The DVD version also lacks all chrome and is replaced by a gray plastic. Also, between MP Prime and the Hasbro 20th pirme, the Hasbro 20th prime's gun is black while the MP is gray.