Thanks for answer and hints!!
The only point is the reality of slalom events in our region. If there are low wind conditions there are 3 main points to consider: 1. earliest planing for start and ofter jibes, 2. upwind capability needed in 8-slaloms because of big wind shifts and 3. keep planing in wind wholes (6-8kn). The final top speed is never the point for winning in our competitions.
That all are the points I thought the iS144 will be the better choice. On the other hand I won a lot of races on 135 and the bigger board was always the better choice.