I love aquariums. I used to have a 50 gallon, but had to get rid of it when we moved into a smaller place.
Freshwater will be cheaper and easier to maintain. There isn't as much to worry about as far as water parameters are concerned, and the fish are cheaper too.
I think starter kits are fine for a beginner. They don't have the greatest components, but you can upgrade any components you need or want as you get more experience and know what you want. Make sure to get a water testing kit, which doesn't usually come with the tank kits, and a bottle of dechlorinator, which might come with the kit but it probably won't last long. Once you get more experience and know what you want you can buy online or craigslist to save money.
If you have the money and room for it, spring for a bigger tank. One reason is that fluctuations in water parameters don't happen as quickly with higher water volume, so you can catch problems before they go too far. The other reason is that you can fit more/bigger fish in a bigger tank.
You should familiarize yourself with the
nitrogen cycle before you get a tank. Knowing about this can prevent fish deaths early on, which can be very frustrating.
And a few last personal tips: 1) Decorations are nice, but too many and they are a pain in the butt to clean around. I would start with fewer decorations rather than more. Sometimes some negative space in a tank can enhance the look. 2) Maintain the tank. Aquariums are great, but if you leave it to get dirty water and algae all over the place, it makes it seem like the whole room is dirty.