A tummy tuck removes fat AND excess skin. Liposuction removes fat ONLY and relies on skin contraction and scarring beneath it to change the body contour.
If your skin is loose, with stretch marks, then a tummy tuck is indicated. The extra skin will not go away with liposuction and will in fact probably get worse.
If your skin is reasonably firm, with good elasticity, and the main problem is excess fat beneath it, then liposuction will work well.
In certain cases in which there is both loose skin and excess fat, both liposuction and tummy tuck can be done at the same time.
Here, I present two cases. The first before and after shows how well liposuction alone works in the right candidate. The second before and after shows the results after tummy tuck.
Liposuction Before and After
Tummy Tuck Before and After