The 101 did have some problems early on, which permanently tarnished its reputation. Currently the 101-650 and 101-750 are good engines, and cheaper to operate than the Turbomeca. I mentioned these two models because they're the only two I have experience with, which were in BK117's.
I don't recall having too many problems with them, other than an occasional torque matching problem due to the airflow modulators on them.
The Arriel is also a great engine. Most models do put out more HP than the 101. We flew the Arriel in our BK117 C-1's as well as in all of our Dauphins. I don't recall too many major problems with them. The only reoccuring item that I remember is that everytime the aircraft (Dauphin) sat in the hangar for a month or two during an inspection, the start-drain valves would leak like a siv on our first runs. No problem though, we just changed them, and that wasn't Turbomeca's fault anyway.
The Arriel is also a true modular engine, so with a trained mech and a few tools it's pretty easy to do a lot of in-house maintenence on them.
All in all I think they're both great engines. It all depends on what the operator needs out of the aircraft, direct operating cost, etc.