"Who is here?"
"Хто там" must be a more common question because why would you ask about who is here right next to you xD
I'd say, "Хто там?" is super common, like when somebody is knocking at your door, and "Хто тут?" is when you are coming back to your bed after riding the fridge at midnight and the room is dark and scary and suddenly you feel some movement... But then it becomes "Хто тут??!!"
I've been typing in the Latin transliterations up until now, but it didn't accept "Xto tut". It's a bit inconvenient for me to put in the Cyrillic text, since I would have to use a converter or memorize the Ukrainian keyboard layout. So...what did I transliterate wrong here?
Perhaps try "Hto tut", since H is the letter that matches Cyrillic Х in pronunciation.
Actually, "Хто тут?" should be translittererad to "Khto tut?" because the letter "Х/х" closest equivalent is "kh", why I do not know. ☺