I can only thoroughly recommend that you stay at Javier's apartment in Mitte if you wish to visit Berlin.
To talk about the flat itself; It is very spacious (at least for 3 guests), the host has provided everything you could need during your stay and it is in a very accessible area of Berlin. The host gave very detailed instructions of how to find and access the flat which worked perfectly for me. I must also mention Javier was very flexible when I wanted to add another guest after I had made my initial booking.
Be mindful that the neighbourhood is quiet and residential, but has shops, supermarkets, bars and places to eat very close by. The transport links are excellent also.