I can't speak for the Medicare side of the equation, but I would consider what the cost/benefit is of pulling expected costs into the known program (your current insurance and known deductible) rather than pushing them into the unknown.
In the end, it does not matter so much on what contributed to the deductible as much as what is paid at a high % after it is maxed out.
Not sure if I made sense, but do a scenario with all the possible expenses mapped to potential coverage. It might be cheaper to max the current plan's deductible than to push things off to late in the year to meet another deductible.
And yes, Part D coverage can be problematic, and can change every year. I've had to help some older relations find a proper plan, only to have the plan change what they covered for the next year. Reset, restart.