Which looks better? - Honestly, this is a personal preference thing. I have sand in my tanks though, i think it looks alot better.
Which do the fish prefer? - This really depends on the fish. At the moment, I can't think of anything off the top of my head that prefers gravel, but i'm sure they are out there. I know my tiger barbs love the sand; they play in it all day. My zebra danios could care less either way.
Which is more expensive? - depends on where you buy your sand and gravel. I bought sand for $3, and could have easily done 2 more 20 gal tanks.
Which would you reccomend? - Which ever you like better, unless your fish/future fish have a set preference (mainly bottom dwellers).
Is it easy to care for? - yup. That was my own little addition. It takes about as much effort as gravel.
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The best thing I can encourage you to do is research, research, reserch and question, question, question. If you hear something, ask why. Learn what your fish need before you go out and buy them or buy the set up.
Also, this is your tank. That said, consider what YOU like and what catches YOUR eye. This is a several year investment, go for what you want. What do you think looks better? Do you need to see some pix of tanks with either?