I've operated both, though not in the corporate role. What follows is based on an IFR version, which I assume you'd have for the corporate role. In no particular order:-
EC135
Avionics, autopilot, maintenance downtime, cabin size, seating capacity, sloping ground capability, build quality, noise, speed.
AS355N
Initial cost, ease of pilot inspection in the field [just], ride quality [the 135 isn't terrible, just rather harder], fuel endurance.
The 135 has a greater payload but also a greater fuel burn so a lot depends on whether you'd need the extra seats at the expense of range.
The 109 is a lovely aircraft but is, as you say, squeezy [only been in one, never flown it] Although the 135 high density seating isn't luxurious, the higher cabin roof gives it a much bigger feeling.
Difficult to comment further without a specific role to analyse, which seems to be the case.