As Mr. X said, you want a tank that has as large of a foot print as possible.
As far as whether or not it's harder, that's more a function of your personality than the animals or the tank. You cannot rush things and you are likely to be looking at an unstocked tank for anywhere from a week to six weeks while the bacteria bed establishes itself. This is where I found that most people lose control, stock too soon or stock with aggressive fish (i.e. damsels) and end up hosing themselves.
You cannot stock a saltwater tank as heavily as you can a freshwater tank.
Generally speaking, the animals in a saltwater tank are more expensive than their freshwater counterparts, so you tend to feel a loss in your wallet a loit harder. You will of course have to spend money on salt. If you do not invest in an RO
unit or but RO
water, you are likely to spend a lot of time fighting cyanobacteria, which most of us will tell you is absolutely no fun at all.
If you want to go reef, then you will also need to invest in a good lighting system. Than single bulb fluorescent that came with the tank isn't going to do much good for any photsynthetic corals or anemones. I think this is the single most expensive startup cost in a reef tank.
I have been able to get away without running any tests on my freshwater tanks for years. (I'll admit here that I am not keeping any sensitve species.) However, you will want to invest in at least pH, ammonia, nitrite, and nitrate kits, along with a hydrometer or better yet a refractometer. Saltwater animals are not as forgiving of fluctuations as freshwater fish. If you don't keep an eye on the parameters, by the time you decide something's wrong, you're way behind the 8 ball.
In summary, I don't think it's hard, but it will cost you more in terms of maintenance, and likely in initial startup as well. I'd also advise that before you take the plunge, you get some advice from forum members regarding what you specifically want to do with your saltwater tank regarding size of tank required, compatibility, etc. You don't want to go theough the work of gettign everything up and going to find out that you're unlikely keep that clown trigger with a mandarin dragonet successfully. Let us know where you want to go, and we'll help you get there.