Completely agree with the last 2 posters. If your intent is to go part 121, then by all means, go part 121. Skip 135. Sure you could log some turbine time, and fly a faster airplane, but there are so many ‘unknowns’ in 135 territory. There’s countless stories of people who have been burned in some way or another by some bullxxxx chief pilot, director of operations, company president, or the customers themselves, etc. I have heard of far more shady practices in that business than anything else.
Chances are, if you go that route, you will probably end up kicking yourself in the rear end for not skipping it altogether and just heading to the airlines. I haven’t done 135, but just about everyone I talk to who has done it, said they wish they would have just skipped it and gone straight to the airlines.
If your gonna go part 121, definitely start prepping for it. Know SID’s and STAR charts like the palm of your hand, know how to read them and brief them quickly, know turbine airplane systems (I.e. APU, hydraulics systems, fuel systems, electrical, bleed air, etc.) v speeds, derived alternate minimums, etc. You don’t need to dive deep into systems because you will do that in ground school, but have a basic understanding of the stuff you will be exposed to going into it. I studied a good bit on this stuff and I feel like it only helped. Gave me less headaches. There’s too many people that don’t do any kind of basic prep going into it that are coming from a part 61/141 CFI background with experience dealing with only piston engines. Can they still get through? Sure, but it will be more painful though.
I recommend picking up a copy of ‘The Turbine Pilot’s flight manual’ or something similar as a sort of generic prep guide. Don’t try to search for or obtain aircraft or airline specific stuff though, whatever company you go with will give you that info. Also recommend sections of ‘everything explained’ as well.