sand or gaval

which is better sand or gaval? i have bettas and guppies

I love the look of sand, But that is just my personal preferance. They say if you got soft belly fish you should use sand. Most of your fish don't spend to much time on the bottom and Gravel would be best for your fish. But your fish dont stir up the bottom so if you like the look of sand go for it. Just remember when you do your clean ups for the sand that you put some kind of cover over the intake on the filter so it doesnt get clogged up with sand.
Do not use an undergravel filter either. That just doesnt go well with the sand.

Oh yah Got to make sure its the normal everyday play sand. The other color sands will have additives in them and will not be good to use.
Also you got to clean the sand before you use it. Its a long process and will take awhile to do.
It took me about 5 and a half hours to clean 100 pounds of sand and it still didnt look clean after that.

I like gravel, but i'm an under gravel filter nut.

i also like gravel.for a power filter this sand can cause a problem...

gravel it is.

what about crush coral that looks nice i have it in some of my tanks

since you have asltwater tank then you can easily keep this crush coral.but for freshwater tank you should not keep that...