Just to clarify terminology, direct commission is a commissioning source and how all physicians and other professionals join the military. You would direct commission whether you joined as a medical student, resident, fellow, or attending. This is in comparison to ROTC, OCS, or the academies as commision sources.
STRAP is a specific incentive program for residents/fellows. Health Professions Special Pay is the incentive available to attendings. The challenge in comparing the two is the commitments are very different. For STRAP it's 12 years if you're doing a 4 year specialty.
STRAP: Drill/AT Pay ($10k/yr x12 years = $120k) + HPRLP ($40k/yr x2 = $80k) + STRAP ($24k/year x4 = $96k)
Total is $300k or $25k per year for 12 years.
Attending incentives vary, I don't know the current rates but let's go with $40k/yr as that is the HPLRP rate if you opt for that over a cash bonus.
Attending: Drill/AT Pay ($10k/yr x8 years = $80k) + Incentive ($40k/yr x 8 years = $320k).
Total of $400k or $50k per year for 8 years.
Still not a fair comparison. Time value of money and the lower opportunity cost of doing your initial military training as a resident rather than an attending makes these much more similar financial options than they seem.
The reality is it doesn't matter how you join the military, they are paying as little as they can to recruit people and the value it takes to recruit people does not vary much between the programs. If it was significantly more expensive to recruit people through one program vs another, they would get rid of the expensive program.
The decision comes down to:
1) Accept that almost no physician is ever making an optimal financial decision when joining the military, at least by the numbers.
2) Most people are likely best served by delaying joining until they are attendings because it keeps commitments to a reasonable length and lets them do a few years of service for a reasonable incentive and then get out.
3) The main benefits of joining earlier are for people who end up making it a 20+ year career. Reaching retirement age earlier in your attending career is a financial perk and earlier integration into military training and opportunities is a career perk.
Edit: In another thread, it looks like you're a PGY3 already, so you would probably only get 1-1.5 years of STRAP and a single HPLRP payment.
STRAP: Drill/AT Pay ($10k/yr x 4.5 years = $45k) + HPRLP ($40k/yr x1 = $40k) + STRAP ($24k/year x2 = $48k)
Total is $133k or $30k per year for 4.5 years.
Attending: Drill/AT Pay ($10k/yr x3 years = $30k) + Incentive ($40k/yr x 3 years = $120k).
Total of $150k or $50k per year for 3 years.
Again, these are rough estimates. Drill/training pay in particularly is probably overestimated in my attempts to account of various perks and pay for any additional training you may choose to pursue. I think joining as an attending is even more likely to be the right answer in your situation.