My advice wouls be to get at least 2 years experience with freshwater tropicals first.
I kept marine aquariums for 20 years and they are stunning. But there are many problems associated with them and the cost of equipemt and livestock can run into thousands.
So that leaves trops. There are some amaizing results achieved with a tropical tank. If you're looking for a lovely underwater garden, go for a nice planted tank with shoals of tetras etc swimming against the lush plants.
As for maintenance, well, a 10% water change once a week, clean the filters once a month, and a daily feed. Of course you may want to trim the plants back a bit and stuff like that but that's all part of the fun. A tank can easily be left for a week whilst you're away and what's better than a drive round some fish shops on a sunday with the kids to buy some fish.
Cons....can be expensive...can also be addictive. If you go for a proper planted tank, you will need good lighting (3 watts of light per gallon of tank water), a nutrient rich substrate and CO2 injection....but believe me it's worth it.
Here's a good forum for you, very beginner friendly with lots of knowledgable members (i'm andy_j on there)
http://www.uk-aquarist.com/index.php and also have a look at the uk aquatic plant society forum...lots of nce pics in the gallery there
http://ukaps.org/forum/
Any questions, please ask.