4:3 stretched gives you wider models, whereas lower FOV gives you wider and taller models, but at the cost of vertical FOV, which is very important in a vertically-oriented game like TF2.
Lower FOV also affects depth perception and makes movement feel slower. Not a problem if you're adjusted, but spectating other players renders the game in their FOV, so you'll be constantly looking through 75 and 90 FOV, making it difficult to maintain familiarity with the feeling of lower FOV.
Some people also experience motion sickness or other ailments in games with low FOV (this being one of the reasons Darn wanted OpenPlugin, IIRC), which doesn't happen with 4:3 stretched.