Reed is smarter and several magnitudes less elastic. Reed also still retains a normal, although strechy biology. Plastic Man does not. He can be torn to bits and as long as someone puts him together, he's perfectly fine. He can also stretch to super super thin levels.
No such thing as a stupid question ....as long as one is honest about it and truly seeking info. Thus, your question is not stupid.
Plastic man and Reed only have FOUR similarities ...the first is that both are comic book characters, the second that they belong in super-groups (the F4 for Reed and JLA for Plastic Man), the third that they are male, and the fourth that they can stretch.
That is where the similarities end and the differences begin.
As mentioned, Reed is very smart, and can come up with a solution to almost any problem when given time and resources. He also seems to have good leadership abilities, and he is quite a good guy (although he can be quite pragmatic at times, taking decisions that while for the greater good can have some not so good effects at the individual level).
Plastic Man, on the other hand, is quite interesting. For one, he is not just 'stretchy' ....he is much much more. For one, he is not even really organic, cannot be affected by even the highest levels of telepathy (one reason why Batman used him against Fernus), is immortal (lived for around 3,000 years when he was sent to the past during the JLA: Obsidian Age arc) and perfectly invulnerable (lived for those 3,000 years while in the form of little pieces scattered all over the ocean floor), and he can change his form into basically anything. Anything he can think of he can change into ....anything. Also, there is really no way of harming Plastic Man after from either using magic to cut him up into molecular pieces, or tech to basically make him unable to remain turgid (and thus become almost liquidish) ....both ways have been used in comics, but none has been able to kill him. Each piece becomes a little PM.
He is basically malleable form made perfect, while Reed is an extremely smart guy who can stretch some distance.
In a stretching competition the two are not even comparable ......Reed would have difficulty stretching to the point of a medium sized city building (if he could do it in the first place), while PM really has no known limits.
The thing about Plastic man is that he is a member of the JLA. Like most of its 'secondary' (people who are not part of the Big 3), they are VERY VERY VERY powerful. People like Animal Man (contact with the Red), Firestorm (a flaming version of the Surfer), Martian Manhunter, Zatanna are all people who could seriously mess up Superman if they wanted to. Actually a lot of the 'secondary' cast could really foul up the world if they wanted to. Plastic Man may not have the offensive capabilities of the rest of them, but he is an immortal invulnerable universal-shape-shifter who is telepathically opaque and has all sorts of tricks up his sleeves. Basically your typical 'global level' JLA 'secondary' cast, who collectively have more power than the 'primary' cast yet are relegated to 'emergency' duty.
Reed is a thin stretchy guy who has a brain that can give Galactus a run for his money (with some PIS ofcourse, but that doesn't take away the fact that Reed is one of the smartest 'normal' people in comics).
Before now i thought he was just the clown version of Reed.
Your implications that Reed's superhero status has little to do with cosmic rays is true. Reed said it himself to Spidey once, he has always considered his stretch power expendable. His real power is his genius.
Plastic Man's also on a completely separate level of shifting. Unlike Elongated Man and Reed, Plastic Man need not keep a basic contiguous, closed shapes. Plastic Man can open holes in his body and can make himself into almost anything, with real, moving parts.
In addition, I've never seen Reed stretch as small and thin as I've seen Plastic Man stretch.