Whatsarunway,
I've flown the BK and BO in EMS, if I had to make a choice I'd take the BK. But if there is an S-76 available I'd jump in that first.
With regards to the BK-117, try to avoid the "A" series helicopters, a bit weak. The B-1 or even better the B-2 have more umph on takeoff. Some of the A's do not have yaw sass I understand, you want to be sure you get a machine with yaw sass. (Yaw sass was added due to the number of med crew getting air sick) Even with yaw sass I find the patient, and often the med crew, get sick in the BK-117. It's a much more sharp edged ride in turblence then any other helicopter except the BO-105. In my experience 10 gusting to 15 feels like 20 gusting to 30 in most other helicopters.
In spite of the room in the back the patients side rubs the side wall on loading so we have had problems with a patient with a broken right arm or leg "fitting" comfortably. Lots of loud comlaint.
I'm also not crazy about the hydraulic system in the BK, same type as the BO. The no 1 system is carring all the loads all the time, then if it fails you have to rely on an automatic switch over to the no 2 system to keep the controls working. There have been a few situations during the switch over where the collective slams down. Good idea when you do the pre-takeoff hydraulic check to leave the collective up a little during the switch over check to see if it drops. An engine overspeed control system has recently been added to the BK too. I have heard of a few situations of it causing a loss of power due to a false sense of an overspeed. Also the overspeed test switches are not "locked out" at 100% N2 so if you move one to test in flight, by mistake, you can have a momentary loss of power, even a shut down in some cases.
Some one else mentioned fuel. Make sure you get the small aux tank, the early A's did not have it. Gives you 180 US gal of fuel. About 2+15 to burn out at 60% torque in the A or B-1 @ 120 kts. Not great for IFR IMO.
Hope this helps you in deciding whether to buy a BK or not. For the cost of a new one you can have a great used 76 which is a much more capable helicopter.
Fly safe.