Hippolite,
The 155 has its share of technical problems like any other helicopter, but we operate from the same location as a whole bunch of 76s (of all marques) and the 155's serviceability record beats the 76 hands down. Its also easier to work on, so downtime overall is far less.
Comparing the 155B1 to the 76C+, the performance is very similar. The new 76C++ may give a performance advantage, but for all the reasons already mentioned on this thread I'd go for the 155.
Incidentally, Bristow operate the 155 for Shell in Nigeria and so are aware of all the "problems", but they are still planning to buy 155s for the Southern North Sea, so it can't be that bad!