AriLcy
I think there was very little difference in operating costs, bmi regional used when first introduced 2 CC on the 145 & 1 on the 135, but over time that became 1 on both until BA acquired bmi and for a short while the 145 operated into LHR but with 2 CC
The engine on the 135 was slightly lower output, but was only a pin change, in fact running it at the same thrust as the 145 reduced fuel burn because it could get to CRZ altitude more quickly.
All of bmir 135/145 came without thrust reversers fitted, in part to save money and weight, but that was another home goal, the route profile was mainly business passengers with carry on luggage, net result was a forward C of G and we all know aircraft with a stab work better with an aft C of G, so as a result bmir became home bases best customer for 22kg blocks of concrete (used to hold the security fencing poles on building sites) normally they would fly around with approx 330KG of these in the boot (this on an aircraft with a MTOM of circa 20 t ) they did acquire some 145 with R/T fitted but these were wire locked out and latter removed for fleet commonality.
The British Midland senior management (joke) had had problems on the Fokker 100 when the R/T was used in anger requiring i think a tail spar inspection and you didn't get any performance credit for them, so they ticked the delete option
when ordering the ERJ.
They also acquired, again i think at least one ex jet Magic with steep approach kit for London City, but needed 2 to make it work, they launched Leeds LCY with a wet lease ATR 42, again midland management thought it would help the route by painting a coconut tree on tail in support of their long haul ops on the 330 to the Caribbean, it didn't and bmi baulked at the rumoured several hundred thousand $ cost of a retro fit steep approach kit, the wet performance out of LCY was ok due lower screen height, but dry was marginal i understand.
Ironically the now CP who was FM on the 146 asked the bmi powers that be many times, "are you absolutely sure you want me to remove the 146 from the AOC for ever, are you sure you'll never want to go into LCY in the future"?
I suspect if rumours are true about the 146 then LCY will be the target, but a E175 would be better…..
Anyway they have a knack of keeping going, lets hope it stays that way