As M. Ted notes, the written notes are just a skeleton of a tune, and the performer supplies the variations in dynamics and timing to make music out of it. A midi made directly by recording what's played on a midi equipped instrument can reproduce what the player did. A midi made from a score can only "play" the notes as written, and may be rather "mechanical."

If you tweak the score to "write it exactly like it's played" you'd get a better midi; but the score would probably be pretty much incomprehensible to someone trying to read and play from the written music. Similarly, when you import a midi recorded from someone's playing on a midi instrument, into a score program, it usually looks like someone spilled a bucket of maggots on the paper.

Some of the notation programs, especially the "high end" ones, allow "notating" midi effects, particularly dynamics and accents, and they're getting better at it - at least according to reports (and advertising claims). In some you can "hide the midi" notations to print the conventionally written score, and still get the tweaks in the exported midi. Many of the middle- to high-end notation programs include "feel filters" that add a little bit of "style" to the midi playbacks made from the notation, largely by supplying rhythmic accents and a little "mistiming;" but at present it's probably still not all that great.

In checking out links for the post above, I note that a few of the more expensive notation programs have added direct "export MP3" and "export WAV" functions, allowing you to "play back" your score with widely available equipment; but the MP3/WAV export would be subject to the same limitations, I suspect, as the midi export.

An increasing number of cheaper programs are now including "scan to score" programs that claim to be able to convert scans (usually .jpg) of a printed sheet to a score editable in the program. (Similar to OCR that converts a scan of a text page into a wordprocessor document, by recognizing the individual characters.) These have been around for a while, but were generally "separate purchase programs." Sometimes they seem to work fairly well, and sometimes not. They should be a lot better now than the last time I test drove one.

A few score writing programs claim to be able to take "audio" input from a microphone and turn it into a written score. Separate programs claiming to do this have been around for a while, and some worked "sort of," but mostly only for a single instrument playing a single melody line. Even chords on a single instrument confused most of them. Current accessories packed with the programs now may be better than what I've seen, but I'd not recommend buying a program based on just this "ability" without test driving to see how well it works - or doesn't.

Lots of programs are cheap enough to try out, and many of the expensive ones have free demo versions. Before committing to one of the expensive ones, it really is important to decide just what you want to do and then find the program that suits your expectations.

John