The 135 can have an aux tank fitted into the boot, don't know if the 109 can, I thought all 109s were IFR?
Also we now have the 135 "e" available...
135 corporate ship cruises 140kts plus.
I think the 109 is so popular due to it's looks and fashion.
A 135 with a snazzy paint-job won't look like Police/Hems...
I am also told the 109 is maintenance heavy whereas the 135 is maintenance light 2hr 100hr service followed by 5day 500hr Service and 10 day 100hr service.
I stand corrected at any time.