Ren'py is a rather popular choice for amateur projects because usually such projects are rather simplistic and do not demand too many functions (I suppose Ren'py might be easier to learn).
(don't quote me on this. Any of this. I don't have any first-hand experience after all)
But, like I said, reception is probably lukewarm at best due to already present misconceptions. I wouldn't worry about it too much though.
As for specifications, you could try here for Ren'py:
Unfortunately, I don't know of a better link for ONScripter other than Insani's site, really.
You may want to consider checking out the Lemmasoft forums for more details:
There are experienced people there who can surely provide more insight into these matters than I can (all my knowledge is second hand and sketchy at best.)
Also, if you're planning a project, this link might be helpful for resources (art, tools, etc):
*edit* oh, didn't mention this earlier, since you didn't ask, but there's a third engine called BLADE. Not nearly as popular as ONScripter or Ren'py, and I know next to nothing about it.