I have a little bit of experience with the Arrius and a fair bit of experience of the P&W 206/207
Both engines are very good in the EC 135. It will probably boil down to the way you want to handle the PBH.
With P&W Eagle Service Plan, the engines (if possible) stay with the airframe this means that when Overhaul time arrives, your engines come out, rentals go in, your engine gets overhauled, when they’re ready, the rentals come out and your original overhauled items get put back in again. The downside of this is that it means two engine removals for the one overhaul event. However, you now start again with a full fresh TBO cycle on the clock.
With TM when the engines reach overhaul, they’ll replace them with another pair and away you go. So only one removal. The downside of this is that the engines you now have may have less than a full TBO cycle on them. What’s more, the cycles for each engine may be different.
There are obvious advantages to each method, you’ll have to decide which one suits you best based on your preference and of course the PBH price.
For what it’s worth, in the 10,000 hours accrued on one 902 and two EC 135s we only had an engine out once other than for overhaul, that was for a FOD event. The 206/207 is a very reliable chunk of engineering.
With the little experience I have of the TM I can say that you'll be satisfied with the engine and the UK TM team are very helpful