If you use a 7ft and 9ft rod that are both the same line weight what's the difference? Is it just the ability to maneuver in tight spots or is there something else? First, we should define short and long rods. Typically, any rod less than 8 feet long is a short rod. Anything longer than 8 feet can be considered long.
You can generally cast farther with a long rod, but the reasons why are somewhat complicated. A longer rod will keep your line higher off the water, especially when wading, and this will help you cast farther. Because it's a longer lever, a longer rod will help you generate more line speed, helping you cast farther. However, a good caster can usually cast farther with a 5' rod than a beginner can with a 9' rod.
Longer rods help with line control when fishing moving water where you're trying to reach over currents. They also generally work better for roll casting. Another thing is that it's often easier for a beginner to feel a longer rod load, which can help them improve their timing.
Shorter rods are generally easier on the wrist, more accurate, and can make landing fish easier after you get them close enough to net. There is also less tip bounce and short rods tend to track the line in a straighter path.
Many folks advocate short rods for fishing in heavy cover or on small streams, but I often find evidence to the contrary. If you're going to be roll casting in that tight cover, you might be better off with an 8'6" rod than a 7' rod. A longer rod in tight cover also gives you the option of dappling/reaching the fly over where the fish is with no cast involved whatsoever. In this case you may be simply rolling, dipping and dapping because there is usually no need for a back cast.
To wrap all that up, I think if you need the rod to reach over currents, roll cast long distances, or cast a long way, a longer rod will be a better choice. If you want to make short accurate casts, cast overhead in heavy cover, or feel light in the hand all day a short rod may be just the ticket.
The following is a list of advantages and disadvantages of short and long fly rods.
Long Rod Advantages
1. More Forgiving: Much more forgiving of poor casting strokes.
2. Back Cast: Enables a higher back cast with ease.
3. Longer Lever: Somewhat easy to lift the line from the water.
4. Line Control: Line mending and control is much easier.
Long Rod Disadvantages
1. Wider Loops: Generally produces wider loops during the cast.
2. Weight: Slightly more heavy that short rods of the same line weight.
3. Awkward: Easy to tangle in close quarters.
4. Landing Fish: More difficult to land fish in the landing zone.
Short Rod Advantages
1. Tighter Loops: Develops high line speed and throws tighter loops.
2. Weight: Short rods weigh less than long rods of the same line weight.
3. Maneuverability: Easier to cast in confined areas.
4. Compact: Easier to transport, especially multi piece short rods.
5. Landing Fish: Easier to land fish in the landing zone; they are closer to you.
Short Rod Disadvantages
1. Less Forgiving: Not forgiving of casting errors.
2. Line Control: Mending fly line is more difficult
3. Hard Work: It takes more effort to be efficient.
4. Shorter Lever: More difficult to lift the line off the water into the back cast.