I can answer your question if not my own 
The Delta store is columnar, I used to think it was row until Hasso corrected one of my blog. The column format is in general a good balance for read/wrote performance under most circumstances.
There is however a row cache that you don't see on any architecture slides that comes into play when high frequency inserts are required and the delta store can't keep up. This is committed to the delta store as soon as possible. I believe it was introduced in SPS06 and it's what gives HANA awesome OLTP performance.
Now you know