"Sounds great!" is used more often for ideas or plans, things you can talk about but can't see or experience yet.
"That's great!" can be used for the same things as "Sounds great!" and everything else.
Since I'm talking about a plan/idea (a vacation) either one is good.
If I did a bunch of cartwheels in front of you, since you can see it happening, you would say "That's great!"
They mean the same thing, you just use them at different times.