That’s easy, there isn’t an F-model Twinstar on this planet that will maintain 7,000 ft in tropical temperatures with one engine out, so what you’d have is twice the risk of a partial power failure that’ll put you in the trees anyway. Except it’s not twice the risk but rather worse because the C20R engine is an abomination. At our local field there’s a Heli-Lynx-converted VFR Twinstar with C20R engines and they’re going through engines at a rate of knots, always running hot and failing power checks. Yes, the modifications to the electrical system are the dog’s whatsits and eliminate a major portion of the maintenance headaches but there are plenty remaining.
The LongRanger is a simple and reliable design. The major “gotcha” in your situation (high density altitude) would be its poor tail rotor authority; get the high DA tail rotor and careful piloting technique will cover the rest. The pilot seats in the LR are far more uncomfortable than the AS355; for the passengers it’s probably a toss-up. The club-type seating in the LR is worse for tourism (couples can’t sit together) but better for VIP. The LR will be about 20 kts slower than the AS355 with equal loads.
I have about 1200 hours in LongRangers, flying from a base at about 6500 MSL; and about 900 hours in AS355F1/2 at or near sea level. As a pilot I enjoyed flying the Twinstar far more than the LongRanger but for an operator Twinstars make no sense unless required by regulations.
Edited to add: not to mention that by choosing Bell you'll avoid having to deal with Eurocopters customer support - or should that be no-support?