"have a seat"
I have two possibilities for saying this:
Toma asiento. -and-
Tenga un asiento.
Which is correct for informal use, or is something else better? Gracias.
I think you're being too literal. A better option would probably be "Siéntete"
The choices you give are word for word translations. They would probably be understood, but not the way the expression would be said by a native speaker.
As far as I know you can also say: Tome asiento, por favor. Siéntese, por favor. Both are formal. Informal of these two would be: Toma asiento. Venga, siéntate. But I am not a native so I am not 100% sure of that
I believe you are looking for "Siéntete, por favor" with the tone of delivery and the "por favor" indicating you are saying it in a "nice" way. If you want to say "Sit down!" then it is "¡Síentete!".
"siéntete" is wrong if you want to mean -have a seat- indeed it's siéntate
Thanks very much, I appreciate your help