Here’s the dillio, ultimately your goal is to fly for a Regional and while everyone talks about the 1500hr total time requirement, some also have a multi-time requirement (and even if they don’t “require” it, it’s something they like to see). Now ATP is one of the few flight schools that does a ton of ME instruction. If you’re going to instruct for ATP you will, without question, end up with hundreds of hours of ME time so why pay for extra ME time when you can GET PAID for it. So what happens if you don’t instruct for ATP? Most of your local flight schools don’t have twins and even if they do they simply don’t do much ME instruction so the chances of getting your time up is slim.
Now truth be told RIGHT NOW most of the Regionals are so short they have removed their ME requirements or lowered it down to the 50 required for the ATP rating, so honestly you should be fine either way. Thing is conditions change and you having the 100hrs would make you more competitive if they do. But personally I don’t believe they will anytime soon. Bottomline, you’ll be fine with the 40.