Here are the pros and cons to using organic (but dead) decorations for your aquarium. Pros for using organic (but dead) decorations include:
Organic (but dead) decorations will be more realistic. Therefore, the end result will be less cheesy for your tank.
Here are the cons to using organic (but dead) decorations:
Organic decorations can occasionally affect tank in minor ways (e.g., driftwood can make water more acidic over time). This is not usually a problem but should be monitored. You definitely should not put driftwood into a saltwater tank.
You'll need to be careful that organic rocks are okay for your tank. Some rocks are inappropriate for freshwater or saltwater tanks. They will leech off harmful minerals. For example, limestone will raise the pH of the tank. Slate, volcanic rock and quartz are completely inert. Both Volcanic rock and quartz can be sharp-edged so watch out. Sandstone should be okay to use as well.
You'll need to be careful that the natural decorations are safe for the fish. Look for sharp edges.
If you have your heart set on a particular look, you may find it harder to pull it off with natural decorations since you're limited to what you can get your hands on. If you go with manufactured items, chances are if you can't find it, you should be able to find somebody who can make it for you.
PROS & CONS for Organic Decorations (Living)
Here are the pros and cons to using organic (but dead) decorations for your aquarium. Pros for using organic (living) decorations include:
Living organic decorations will provide you with the most realistic and possibly interesting results.
Here are the cons to using organic (living) decorations:
Keeping real organic decorations can be an expensive venture. You'll need to replace them when they reach their natural life span.
Natural organic decorations are harder to care for. When they die, if you don't keep tabs on them, they can really make problems for your tank (anywhere from just making it incredibly dirty to actually releasing dangerous toxins into the water).
Your fish may consume them!
In an interesting twist, you may find your decorating plans compromised by natural choices. Selection may be limited in terms of natural colors and sizes.
Tips for Decorating Your Tank like a Pro
The key to decorating your tank so that it looks professionally designed is to have a plan. Consider first what look you're going for. You can either pick a theme or decide to go eclectic. Decide what types of decorations you want to use. Figure out how much space you have in your tank for your decorations.
Consider getting some kind of background for your tank. This can be a picture, or just a color such as a graded blue or a simple black. It will help hide equipment you may have in the back and keep anything else in the background hidden from your view. A background often makes the difference between a professional look and an amateur at work.
There are some great books out there that can help you decorate your own tank. Here are my favorites:
Aquarium Style by Matthew Christian. Published in 2000, list price $24.95. Great ideas!
Aquarium Designs: Inspired by Nature by Peter Hiscock. Published in 2004, list price $27.95. Peter Hiscock KNOWS his plants.
One last thought I'd like to leave you with is that you don't have to go with ONLY real decorations or manufactured decorations. I have seen plenty of tanks that have successfully combined real (living), organic (but dead), and manufactured (some made to look real but many were not) decorative elements that made for great tanks.